Timestamp calibration of the 3d camera with epipolar line laser point scanning

ABSTRACT

Using the same image sensor to capture a two-dimensional (2D) image and three-dimensional (3D) depth measurements for a 3D object. A laser point-scans the surface of the object with light spots, which are detected by a pixel array in the image sensor to generate the 3D depth profile of the object using triangulation. Each row of pixels in the pixel array forms an epipolar line of the corresponding laser scan line. Timestamping provides a correspondence between the pixel location of a captured light spot and the respective scan angle of the laser to remove any ambiguity in triangulation. An Analog-to-Digital Converter (ADC) in the image sensor operates as a Time-to-Digital (TDC) converter to generate timestamps. A timestamp calibration circuit is provided on-board to record the propagation delay of each column of pixels in the pixel array and to provide necessary corrections to the timestamp values generated during 3D depth measurements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/973,709, filed Dec. 17, 2015, which is a continuation-in-part of andclaims the priority benefit under 35 U.S.C. § 120 of the U.S. patentapplication Ser. No. 14/842,822 filed on Sep. 1, 2015, which claims thebenefit of commonly assigned U.S. Provisional Application No. 62/150,252filed on Apr. 20, 2015 and U.S. Provisional Application No. 62/182,404,filed on Jun. 19, 2015, the disclosures of all of these applications areincorporated herein by reference in their entireties. This applicationalso claims the priority benefit under 35 U.S.C. § 119(e) of U.S.Provisional Application No. 62/253,123 filed on Nov. 9, 2015, thedisclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to image sensors. Morespecifically, and not by way of limitation, particular embodiments ofthe inventive aspects disclosed in the present disclosure are directedto timestamp calibration in a triangulation-based system and method ofdepth measurements on a three-dimensional (3D) object using a laserpoint scan and a Complementary Metal Oxide Semiconductor (CMOS) imagesensor, which is also used for two-dimensional (2D) imaging of the 3Dobject.

BACKGROUND

Three-dimensional (3D) imaging systems are increasingly being used in awide variety of applications such as, for example, industrialproduction, video games, computer graphics, robotic surgeries, consumerdisplays, surveillance videos, 3D modeling, real estate sales, and soon.

Existing 3D imaging technologies may include, for example, thetime-of-flight (TOF) based range imaging, stereo vision systems, andstructured light (SL) methods.

In the TOF method, distance to a 3D object is resolved based on theknown speed of light—by measuring the round-trip time it takes for alight signal to travel between a camera and the 3D object for each pointof the image. A TOF camera may use a scannerless approach to capture theentire scene with each laser or light pulse. Some example applicationsof the TOF method may include advanced automotive applications such asactive pedestrian safety or pre-crash detection based on distance imagesin real time, to track movements of humans such as during interactionwith games on video game consoles, in industrial machine vision toclassify objects and help robots find the items such as items on aconveyor belt, and so on.

In stereoscopic imaging or stereo vision systems, two cameras—displacedhorizontally from one another—are used to obtain two differing views ona scene or a 3D object in the scene. By comparing these two images, therelative depth information can be obtained for the 3D object. Stereovision is highly important in fields such as robotics, to extractinformation about the relative position of 3D objects in the vicinity ofautonomous systems/robots. Other applications for robotics includeobject recognition, where stereoscopic depth information allows arobotic system to separate occluding image components, which the robotmay otherwise not be able to distinguish as two separate objects—such asone object in front of another, partially or fully hiding the otherobject. 3D stereo displays are also used in entertainment and automatedsystems.

In the SL approach, the 3D shape of an object may be measured usingprojected light patterns and a camera for imaging. In the SL method, aknown pattern of light—often grids or horizontal bars or patterns ofparallel stripes—is projected onto a scene or a 3D object in the scene.The projected pattern may get deformed or displaced when striking thesurface of the 3D object. Such deformation may allow an SL vision systemto calculate the depth and surface information of the object. Thus,projecting a narrow band of light onto a 3D surface may produce a lineof illumination that may appear distorted from other perspectives thanthat of the projector, and can be used for geometric reconstruction ofthe illuminated surface shape. The SL-based 3D imaging maybe used indifferent applications such as, for example, by a police force tophotograph fingerprints in a 3D scene, inline inspection of componentsduring a production process, in health care for live measurements ofhuman body shapes or the micro structures of human skin, and the like.

SUMMARY

In one embodiment, the present disclosure is directed to a method thatcomprises: (i) performing a one-dimensional (1D) point scan of athree-dimensional (3D) object along a scanning line using a lightsource, wherein the point scan projects a sequence of light spots on asurface of the 3D object; (ii) selecting a row of pixels in an imagesensor, wherein the image sensor has a plurality of pixels arranged in atwo-dimensional (2D) array forming an image plane, and wherein theselected row of pixels forms at least a portion of an epipolar line ofthe scanning line on the image plane; (iii) for a pixel in the selectedrow of pixels, sensing a pixel-specific detection of a correspondinglight spot in the sequence of light spots; (iv) in response to sensingthe pixel-specific detection of the corresponding light spot, generatinga timestamp value for the corresponding light spot; (v) for a column inthe 2D array associated with the pixel in the selected row of pixels,applying a column-specific correction value to the timestamp value toobtain a corrected timestamp value, wherein the column-specificcorrection value represents a column-specific propagation delay betweensensing of the pixel-specific detection and when a pixel-specific outputof the pixel in the selected row of pixels reaches a pre-definedthreshold; and (vi) determining a distance to the corresponding lightspot on the surface of the 3D object based at least on the correctedtimestamp value and on a scan angle used by the light source forprojecting the corresponding light spot.

In another embodiment, the present disclosure is directed to an imagingunit that comprises: (i) a light source operative to perform a 1D pointscan of a 3D object along a scanning line, wherein the point scanprojects a sequence of light spots on a surface of the 3D object; and(ii) an image sensor unit. The image sensor unit includes: (i) aplurality of pixels arranged in a 2D pixel array forming an image plane,wherein a row of pixels in the 2D pixel array forms at least a portionof an epipolar line of the scanning line, wherein each pixel in the rowof pixels is associated with a respective column in the 2D pixel array,and wherein each pixel in the row of pixels is operative to detect acorresponding light spot in the sequence of light spots; (ii) aplurality of Analog-to-Digital Converter (ADC) units, wherein each ADCunit is associated with a respective pixel in the row of pixels and isoperative to generate a pixel-specific timestamp value for therespective pixel in response to a pixel-specific detection of thecorresponding light spot by the respective pixel; and (iii) a processingunit coupled to the plurality of ADC units. In the image sensor unit,the processing unit is operative to perform the following: (i) for acolumn in the 2D array associated with the respective pixel in the rowof pixels, apply a column-specific correction value to thepixel-specific timestamp value to obtain a corrected timestamp value,wherein the column-specific correction value represents acolumn-specific propagation delay between the pixel-specific detectionand when a pixel-specific output of the respective pixel reaches apre-defined threshold; and (ii) determine a distance to thecorresponding light spot on the surface of the 3D object based at leaston the corrected timestamp value and on a scan angle used by the lightsource for projecting the corresponding light spot.

In a further embodiment, the present disclosure is directed to a system,which comprises: (i) a light source; (ii) a plurality of pixels arrangedin a 2D pixel array; (iii) a plurality of ADC units; (iv) a memory forstoring program instructions; and (v) a processor coupled to the memoryand to the plurality of ADC units. In the system, the light source isoperative to perform a 1D point scan of a 3D object along a scanningline, wherein the point scan projects a sequence of light spots on asurface of the 3D object. Also in the system, the 2D pixel array formsan image plane, wherein a row of pixels in the 2D pixel array forms atleast a portion of an epipolar line of the scanning line. Each pixel inthe row of pixels is associated with a respective column in the 2D pixelarray in the system, and wherein each pixel in the row of pixels isoperative to detect a corresponding light spot in the sequence of lightspots. Each ADC unit in the system is associated with a respective pixelin the row of pixels and is operative to generate a pixel-specifictimestamp value for the respective pixel in response to a pixel-specificdetection of the corresponding light spot by the respective pixel. Inthe system, the processor is configured to execute the programinstructions (stored in the memory), whereby the processor is operativeto perform the following: (i) for a column in the 2D pixel arrayassociated with the respective pixel in the row of pixels, apply acolumn-specific correction value to the pixel-specific timestamp valueto obtain a corrected timestamp value, wherein the column-specificcorrection value represents a column-specific propagation delay betweenthe pixel-specific detection and when a pixel-specific output of therespective pixel reaches a pre-defined threshold; and (ii) determine adistance to the corresponding light spot on the surface of the 3D objectbased at least on the corrected timestamp value and on a scan angle usedby the light source for projecting the corresponding light spot.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following section, the inventive aspects of the presentdisclosure will be described with reference to exemplary embodimentsillustrated in the figures, in which:

FIG. 1 shows a highly simplified, partial layout of a system accordingto one embodiment of the present disclosure;

FIG. 2 illustrates an exemplary operational layout of the system in FIG.1 according to one embodiment of the present disclosure;

FIG. 3 depicts an exemplary flowchart showing how 3D depth measurementsmay be performed according to one embodiment of the present disclosure;

FIG. 4 is an exemplary illustration of how a point scan may be performedfor 3D depth measurements according to one embodiment of the presentdisclosure;

FIG. 5 illustrates an exemplary time-stamping for scanned light spotsaccording to one embodiment of the present disclosure;

FIG. 6 shows exemplary circuit details of the 2D pixel array and aportion of the associated processing circuits in the image processingunit of the image sensor in FIGS. 1-2 according to one embodiment of thepresent disclosure;

FIG. 7A is an exemplary layout of an image sensor unit according to oneembodiment of the present disclosure;

FIG. 7B shows architectural details of an exemplary CDS+ADC unit for 3Ddepth measurement according to one embodiment of the present disclosure;

FIG. 8 is a timing diagram that shows exemplary timing of differentsignals in the system of FIGS. 1-2 to generate timestamp-basedpixel-specific outputs in a 3D mode of operation according to particularembodiments of the present disclosure;

FIG. 9 shows an exemplary flowchart illustrating how timestamp valuesmay be corrected during 3D depth measurements according to oneembodiment of the present disclosure;

FIG. 10 is an exemplary layout of a timestamp calibration unit accordingto one embodiment of the present disclosure;

FIG. 11 is a timing diagram that shows exemplary timing of differentsignals associated with the calibration unit of FIG. 11 to determinecolumn-specific propagation delays according to particular embodimentsof the present disclosure; and

FIG. 12 depicts an overall layout of the system in FIGS. 1-2 accordingto one embodiment of the present disclosure.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the disclosure.However, it will be understood by those skilled in the art that thedisclosed inventive aspects may be practiced without these specificdetails. In other instances, well-known methods, procedures, componentsand circuits have not been described in detail so as not to obscure thepresent disclosure. Additionally, the described inventive aspects can beimplemented to perform low power, 3D depth measurements in any imagingdevice or system, including, for example, a smartphone, a User Equipment(UE), a laptop computer, and the like.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present disclosure. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” or“according to one embodiment” (or other phrases having similar import)in various places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments. Also, depending on the context of discussionherein, a singular term may include its plural forms and a plural termmay include its singular form. Similarly, a hyphenated term (e.g.,“two-dimensional,” “pre-determined”, “pixel-specific,” etc.) may beoccasionally interchangeably used with its non-hyphenated version (e.g.,“two dimensional,” “predetermined”, “pixel specific,” etc.), and acapitalized entry (e.g., “Counter Clock,” “Row Select,” “PIXOUT,” etc.)may be interchangeably used with its non-capitalized version (e.g.,“counter clock,” “row select,” “pixout,” etc.). Such occasionalinterchangeable uses shall not be considered inconsistent with eachother.

It is noted at the outset that the terms “coupled,” “operativelycoupled,” “connected”, “connecting,” “electrically connected,” etc., maybe used interchangeably herein to generally refer to the condition ofbeing electrically/electronically connected in an operative manner.Similarly, a first entity is considered to be in “communication” with asecond entity (or entities) when the first entity electrically sendsand/or receives (whether through wireline or wireless means) informationsignals (whether containing address, data, or control information)to/from the second entity regardless of the type (analog or digital) ofthose signals. It is further noted that various figures (includingcomponent diagrams) shown and discussed herein are for illustrativepurpose only, and are not drawn to scale. Similarly, various waveformsand timing diagrams are shown for illustrative purpose only.

The terms “first,” “second,” etc., as used herein, are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.) unless explicitly defined assuch. Furthermore, the same reference numerals may be used across two ormore figures to refer to parts, components, blocks, circuits, units, ormodules having the same or similar functionality. However, such usage isfor simplicity of illustration and ease of discussion only; it does notimply that the construction or architectural details of such componentsor units are the same across all embodiments or such commonly-referencedparts/modules are the only way to implement the teachings of particularembodiments of the present disclosure.

It is observed here that the earlier-mentioned 3D technologies have manydrawbacks. For example, a TOF-based 3D imaging system may require highpower to operate optical or electrical shutters. These systems typicallyoperate over a range of few meters to several tens of meters, but theresolution of these systems decreases for measurements over shortdistances, thereby making 3D imaging within a distance of about onemeter almost impractical. Hence, a TOF system may not be desirable forcell phone-based camera applications, where pictures are pre-dominantlytaken at close distances. A TOF sensor may also require special pixelswith big pixel sizes, usually larger than 7 μm. These pixels also may bevulnerable to ambient light.

The stereoscopic imaging approach generally works only with texturedsurfaces. It has high computational complexity because of the need tomatch features and find correspondences between the stereo pair ofimages of an object. This requires high system power, which is not adesirable attribute where power conservation is needed, such as insmartphones. Furthermore, stereo imaging requires two regular, high bitresolution sensors along with two lenses, making the entire assemblyunsuitable for applications in portable devices, like cell phones ortablets, where device real estate is at a premium.

The SL approach introduces distance ambiguity, and also requires highsystem power. For 3D depth measurements, the SL method may need multipleimages with multiple patterns—all of these increase computationalcomplexity and power consumption. Furthermore, the SL imaging may alsorequire regular image sensors with high bit resolution. Thus, astructured light-based system may not be suitable for low-cost,low-power, compact image sensors in smartphones.

In contrast to the above-mentioned 3D technologies, particularembodiments of the present disclosure provide for implementing a lowpower, 3D imaging system on portable electronic devices such assmartphones, tablets, UEs, and the like. A 2D imaging sensor as perparticular embodiments of the present disclosure can capture both 2D RGB(Red, Green, Blue) images and 3D depth measurements with visible lightlaser scanning. It is noted here that although the following discussionmay frequently mention the visible light laser as a light source forpoint-scans and a 2D RGB sensor as an image/light capture device, suchmention is for the purpose of illustration and consistency of discussiononly. The visible laser and RGB sensor based examples discussed belowmay find applications in low-power, consumer-grade mobile electronicdevices with cameras such as, for example, smartphones, tablets, or UEs.However, it is understood that the teachings of the present disclosureare not limited to the visible laser-RGB sensor based examples mentionedbelow. Rather, according to particular embodiments of the presentdisclosure, the point scan-based 3D depth measurements and the ambientlight rejection methodology may be performed using many differentcombinations of 2D sensors and laser light sources (for point scans)such as, for example: (i) a 2D color (RGB) sensor with a visible lightlaser source, in which the laser source may be a red (R), green (G), orblue (B) light laser, or a laser source producing a combination of theselights; (ii) a visible light laser with a 2D RGB color sensor having anInfrared (IR) cut filter; (iii) a Near Infrared (NIR) laser with a 2D IRsensor; (iv) an NIR laser with a 2D NIR sensor; (v) an NIR laser with a2D RGB sensor (without an IR cut filter); (vi) an NIR laser with a 2DRGB sensor (without an NIR cut filter); (vii) a 2D RGB-IR sensor withvisible or NIR laser; (viii) a 2D RGBW (red, green, blue, white) sensorwith either visible or NIR laser; and so on.

During 3D depth measurements, the entire sensor may operate as a binarysensor in conjunction with the laser scan to reconstruct 3D content. Inparticular embodiments, the pixel size of the sensor can be as small as1 μm. Furthermore, due to lower bit resolution, the Analog-to-DigitalConverter (ADC) units in the image sensor according to particularembodiments of the present disclosure may require significantly muchlower processing power than that is needed for high bit resolutionsensors in traditional 3D imaging systems. Because of the need for lessprocessing power, the 3D imaging module according to present disclosuremay require low system power and, hence, may be quite suitable forinclusion in low power devices like smartphones.

In particular embodiments, the present disclosure uses triangulation andpoint scans with a laser light source for 3D depth measurements with agroup of line sensors. The laser scanning plane and the imaging planeare oriented using epipolar geometry. An image sensor according to oneembodiment of the present disclosure may use timestamps to removeambiguity in the triangulation approach, thereby reducing the amount ofdepth computations and system power. An on-board timestamp calibrationunit may be provided to compensate for timestamp jitters and errorscaused by variations in the signal propagation delays associated withthe readout chains of different columns of sensor pixels. The same imagesensor—that is, each pixel in the image sensor—may be used in the normal2D (RGB color or non-RGB) imaging mode as well as in the 3D laser scanmode. However, in the laser scan mode, the resolution of the ADCs in theimage sensor is reduced to a binary output (1-bit resolution only),thereby improving the readout speed and reducing power consumption—forexample, due to switching in the ADC units—in the chip incorporating theimage sensor and associated processing units. Furthermore, the pointscan approach may allow the system to take all measurements in one pass,thereby reducing the latency for depth measurements and reducing motionblur.

As noted before, in particular embodiments, the entire image sensor maybe used for routine 2D RGB color imaging using, for example, ambientlight, as well as for 3D depth imaging using visible laser scan. Suchdual use of the same camera unit may save space and cost for mobiledevices. Furthermore, in certain applications, the user of visible laserfor 3D applications may be better for a user's eye safety as compared toa Near Infrared (NIR) laser. The sensor may have higher quantumefficiency at visible spectrum that at the NIR spectrum, leading tolower power consumption of the light source. In one embodiment, thedual-use image sensor may work in a linear mode of operation for 2Dimaging—as a regular 2D sensor. However, for 3D imaging, the sensor maywork in linear mode under moderate lighting condition and in logarithmicmode under strong ambient light to facilitate continued use of thevisible laser source through rejection of the strong ambient light.Furthermore, ambient light rejection may be needed in case of an NIRlaser as well, for example, when the bandwidth of the pass band of anIR-cut filter employed with an RGB sensor is not narrow enough.

FIG. 1 shows a highly simplified, partial layout of a system 15according to one embodiment of the present disclosure. As shown, thesystem 15 may include an imaging module 17 coupled to and incommunication with a processor or host 19. The system 15 may alsoinclude a memory module 20 coupled to the processor 19 to storeinformation content such as, for example, image data received from theimaging module 17. In particular embodiments, the entire system 15 maybe encapsulated in a single Integrated Circuit (IC) or chip.Alternatively, each of the modules 17, 19, and 20 may be implemented ina separate chip. Furthermore, the memory module 20 may include more thanone memory chip, and the processor module 19 may comprise of multipleprocessing chips as well. In any event, the details about packaging ofthe modules in FIG. 1 and how they are fabricated or implemented—in asingle chip or using multiple discrete chips—are not relevant to thepresent discussion and, hence, such details are not provided herein.

The system 15 may be any low power, electronic device configured for 2Dand 3D camera applications as per teachings of the present disclosure.The system 15 may be portable or non-portable. Some examples of theportable version of the system 15 may include popular consumerelectronic gadgets such as, for example, a mobile device, a cellphone, asmartphone, a User Equipment (UE), a tablet, a digital camera, a laptopor desktop computer, an electronic smartwatch, a Machine-to-Machine(M2M) communication unit, a Virtual Reality (VR) equipment or module, arobot, and the like. On the other hand, some examples of thenon-portable version of the system 15 may include a game console in avideo arcade, an interactive video terminal, an automobile, a machinevision system, an industrial robot, a VR equipment, a driver-sidemounted camera in a car (for example, to monitor whether the driver isawake or not), and so on. The 3D imaging functionality provided as perteachings of the present disclosure may be used in many applicationssuch as, for example, virtual reality applications on a virtual realityequipment, online chatting/gaming, 3D texting, searching an online orlocal (device-based) catalog/database using a product's 3D image toobtain information related to the product (for example, calorie contentof a piece of food item), robotics and machine vision applications,automobile applications such as autonomous driving applications, and thelike.

In particular embodiments of the present disclosure, the imaging module17 may include a light source 22 and an image sensor unit 24. Asdiscussed in more detail with reference to FIG. 2 below, in oneembodiment, the light source 22 may be a visible laser. In otherembodiments, the light source may be an NIR laser. The image sensor unit24 may include a pixel array and ancillary processing circuits as shownin FIG. 2 and also discussed below.

In one embodiment, the processor 19 may be a CPU, which can be a generalpurpose microprocessor. In the discussion herein, the terms “processor”and “CPU” may be used interchangeably for ease of discussion. However,it is understood that, instead of or in addition to the CPU, theprocessor 19 may contain any other type of processors such as, forexample, a microcontroller, a Digital Signal Processor (DSP), a GraphicsProcessing Unit (GPU), a dedicated Application Specific IntegratedCircuit (ASIC) processor, and the like. Furthermore, in one embodiment,the processor/host 19 may include more than one CPU, which may beoperative in a distributed processing environment. The processor 19 maybe configured to execute instructions and to process data according to aparticular Instruction Set Architecture (ISA) such as, for example, anx86 instruction set architecture (32-bit or 64-bit versions), a PowerPC®ISA, or a MIPS (Microprocessor without Interlocked Pipeline Stages)instruction set architecture relying on RISC (Reduced Instruction SetComputer) ISA. In one embodiment, the processor 19 may be a System onChip (SoC) having functionalities in addition to a CPU functionality.

In particular embodiments, the memory module 20 may be a Dynamic RandomAccess Memory (DRAM) such as, for example, a Synchronous DRAM (SDRAM),or a DRAM-based Three Dimensional Stack (3DS) memory module such as, forexample, a High Bandwidth Memory (HBM) module, or a Hybrid Memory Cube(HMC) memory module. In other embodiments, the memory module 20 may be aSolid State Drive (SSD), a non-3DS DRAM module, or any othersemiconductor-based storage system such as, for example, a Static RandomAccess Memory (SRAM), a Phase-Change Random Access Memory (PRAM orPCRAM), a Resistive Random Access Memory (RRAM or ReRAM), aConductive-Bridging RAM (CBRAM), a Magnetic RAM (MRAM), a Spin-TransferTorque MRAM (STT-MRAM), and the like.

FIG. 2 illustrates an exemplary operational layout of the system 15 inFIG. 1 according to one embodiment of the present disclosure. The system15 may be used to obtain depth information (along the Z-axis) for a 3Dobject, such as the 3D object 26, which may be an individual object oran object within a scene (not shown). In one embodiment, the depthinformation may be calculated by the processor 19 based on the scan datareceived from the image sensor unit 24. In another embodiment, the depthinformation may be calculated by the image sensor unit 24 itself suchas, for example, in case of the image sensor unit in the embodiment ofFIG. 7A. In particular embodiments, the depth information may be used bythe processor 19 as part of a 3D user interface to enable the user ofthe system 15 to interact with the 3D image of the object or use the 3Dimage of the object as part of games or other applications running onthe system 15. The 3D imaging as per teachings of the present disclosuremay be used for other purposes or applications as well, and may beapplied to substantially any scene or 3D objects.

In FIG. 2, the X-axis is taken to be the horizontal direction along thefront of the device 15, the Y-axis is the vertical direction (out of thepage in this view), and the Z-axis extends away from the device 15 inthe general direction of the object 26 being imaged. For the depthmeasurements, the optical axes of the modules 22 and 24 may be parallelto the Z-axis. Other optical arrangements may be used as well toimplement the principles described herein, and these alternativearrangements are considered to be within the scope of the presentdisclosure.

The light source module 22 may illuminate the 3D object 26 as shown byexemplary arrows 28-29 associated with corresponding dotted lines 30-31representing an illumination path of a light beam or optical radiationthat may be used to point scan the 3D object 26 within an optical fieldof view. A line-by-line point scan of the object surface may beperformed using an optical radiation source, which, in one embodiment,may be a laser light source 33 operated and controlled by a lasercontroller 34. A light beam from the laser source 33 may be pointscanned—under the control of the laser controller 34—in the X-Ydirection across the surface of the 3D object 26 via projection optics35. The point scan may project light spots on the surface of the 3Dobject along a scan line, as discussed in more detail with reference toFIGS. 4-5 below. The projection optics may be a focusing lens, aglass/plastics surface, or other cylindrical optical element thatconcentrates laser beam from the laser 33 as a point or spot on thatsurface of the object 26. In the embodiment of FIG. 2, a convexstructure is shown as a focusing lens 35. However, any other suitablelens design may be selected for projection optics 35. The object 26 maybe placed at a focusing location where illuminating light from the lightsource 33 is focused by the projection optics 35 as a light spot. Thus,in the point scan, a point or narrow area/spot on the surface of the 3Dobject 26 may be illuminated sequentially by the focused light beam fromthe projection optics 35.

In particular embodiments, the light source (or illumination source) 33may be a diode laser or a Light Emitting Diode (LED) emitting visiblelight, an NIR laser, a point light source, a monochromatic illuminationsource (such as, for example, a combination of a white lamp and amonochromator) in the visible light spectrum, or any other type of laserlight source. The laser 33 may be fixed in one position within thehousing of the device 15, but may be rotatable in X-Y directions. Thelaser 33 may be X-Y addressable (for example, by the laser controller34) to perform point scan of the 3D object 26. In one embodiment, thevisible light may be substantially green light. The visible lightillumination from the laser source 33 may be projected onto the surfaceof the 3D object 26 using a mirror (not shown), or the point scan may becompletely mirror-less. In particular embodiments, the light sourcemodule 22 may include more or less components than those shown in theexemplary embodiment of FIG. 2.

In the embodiment of FIG. 2, the light reflected from the point scan ofthe object 26 may travel along a collection path indicated by arrows36-37 and dotted lines 38-39. The light collection path may carryphotons reflected from or scattered by the surface of the object 26 uponreceiving illumination from the laser source 33. It is noted here thatthe depiction of various propagation paths using solid arrows and dottedlines in FIG. 2 (and also in FIGS. 4-5, as applicable) is forillustrative purpose only. The depiction should not be construed toillustrate any actual optical signal propagation paths. In practice, theillumination and collection signal paths may be different from thoseshown in FIG. 2, and may not be as clearly-defined as in theillustration in FIG. 2.

The light received from the illuminated object 26 may be focused ontoone or more pixels of a 2D pixel array 42 via collection optics 44 inthe image sensor unit 24. Like the projection optics 35, the collectionoptics 44 may be a focusing lens, a glass/plastics surface, or othercylindrical optical element that concentrates the reflected lightreceived from the object 26 onto one or more pixels in the 2D array 42.In the embodiment of FIG. 2, a convex structure is shown as a focusinglens 44. However, any other suitable lens design may be selected forcollection optics 44. Furthermore, for ease of illustration, only a 3×3pixel array is shown in FIG. 2 (and also in FIG. 6). However, it isunderstood that, modern pixel arrays contain thousands or even millionsof pixels. The pixel array 42 may be an RGB pixel array, in whichdifferent pixels may collect light signals of different colors. Asmentioned before, in particular embodiments, the pixel array 42 may beany 2D sensor such as, for example, a 2D RGB sensor with IR cut filter,a 2D IR sensor, a 2D NIR sensor, a 2D RGBW sensor, a 2D RWB (Red, White,Blue) sensor, a multi-layer CMOS organic sensor, a 2D RGB-IR sensor, andthe like. As discussed in more detail later, the system 15 may use thesame pixel array 42 for 2D RGB color imaging of the object 26 (or ascene containing the object) as well as for 3D imaging (involving depthmeasurements) of the object 26. Additional architectural details of thepixel array 42 are discussed later with reference to FIG. 6.

The pixel array 42 may convert the received photons into correspondingelectrical signals, which are then processed by the associated imageprocessing unit 46 to determine the 3D depth image of the object 26. Inone embodiment, the image processing unit 46 may use triangulation fordepth measurements. The triangulation approach is discussed later withreference to FIG. 4. The image processing unit 46 may also includerelevant circuits for controlling the operation of the pixel array 42.Exemplary image processing and control circuits are illustrated in FIGS.7A-7B, which are discussed later below.

The processor 19 may control the operations of the light source module22 and the image sensor unit 24. For example, the system 15 may have amode switch (not shown) controllable by the user to switch from 2Dimaging mode to 3D imaging mode. When the user selects the 2D imagingmode using the mode switch, the processor 19 may activate the imagesensor unit 24, but may not activate the light source module 22 because2D imaging may use ambient light. On the other hand, when the userselects 3D imaging using the mode switch, the processor 19 may activateboth of the modules 22, 24 (as discussed below). The processed imagedata received from the image processing unit 46 may be stored by theprocessor 19 in the memory 20. The processor 19 may also display theuser-selected 2D or 3D image on a display screen (not shown) of thedevice 15. The processor 19 may be programmed in software or firmware tocarry out various processing tasks described herein. Alternatively oradditionally, the processor 19 may comprise programmable hardware logiccircuits for carrying out some or all of its functions. In particularembodiments, the memory 20 may store program code, look-up tables,and/or interim computational results to enable the processor 19 to carryout its functions.

FIG. 3 depicts an exemplary flowchart 50 showing how 3D depthmeasurements may be performed according to one embodiment of the presentdisclosure. Various steps illustrated in FIG. 3 may be performed by asingle module or a combination of modules or system components in thesystem 15. In the discussion herein, by way of an example only, specifictasks are described as being performed by specific modules or systemcomponents. Other modules or system components may be suitablyconfigured to perform such tasks as well.

In FIG. 3, at block 52, the system 15 (more specifically, the processor19) may perform a one-dimensional (1D) point scan of a 3D object, suchas the object 26 in FIG. 2, along a scanning line using a light source,such as the light source module 22 in FIG. 2. As part of the point scan,the light source module 22 may be configured, for example, by theprocessor 19, to project a sequence of light spots on a surface of the3D object 26 in a line-by-line manner. At block 54, the pixel processingunit 46 in the system 15 may select a row of pixels in an image sensor,such as the 2D pixel array 42 in FIG. 2. The image sensor 42 has aplurality of pixels arranged in a 2D array forming an image plane, and,in one embodiment, the selected row of pixels forms an epipolar line ofthe scanning line (at block 52) on the image plane. A brief discussionof epipolar geometry is provided below with reference to FIG. 4. Atblock 56, the pixel processing unit 46 may be operatively configured bythe processor 19 to detect each light spot using a corresponding pixelin the row of pixels. It is observed here that light reflected from anilluminated light spot may be detected by a single pixel or more thanone pixel such as, for example, when the light reflected from theilluminated spot gets focused by the collection optics 44 onto two ormore adjacent pixels. On the other hand, it may be possible that lightreflected from two or more light spots may be collected at a singlepixel in the 2D array 42. The timestamp-based approach discussed belowremoves depth calculation-related ambiguities resulting from imaging oftwo different spots by the same pixel or imaging of a single spot by twodifferent pixels. At block 58, the image processing unit 46—as suitablyconfigured by the processor 19—may generate a pixel-specific output inresponse to a pixel-specific detection (at block 56) of a correspondinglight spot in the sequence of light spots (in the point scan at block52). Consequently, at block 60, the image processing unit 46 maydetermine the 3D distance (or depth) to the corresponding light spot onthe surface of the 3D object based at least on the pixel-specific output(at block 58) and on a scan angle used by the light source forprojecting the corresponding light spot (at block 52). The depthmeasurement is discussed in more detail with reference to FIG. 4.

FIG. 4 is an exemplary illustration of how a point scan may be performedfor 3D depth measurements according to one embodiment of the presentdisclosure. In FIG. 4, the X-Y rotational capabilities of the lasersource 33 are illustrated using the arrows 62, 64 depicting the laser'sangular motions in the X-direction (having angle “β”) and in theY-direction (having angle “α”). In one embodiment, the laser controller34 may control the X-Y rotation of the laser source 33 based on scanninginstructions/input received from the processor 19. For example, when theuser selects 3D imaging mode, the processor 19 may instruct the lasercontroller 34 to initiate 3D depth measurements of the object surfacefacing the projection optics 35. In response, the laser controller 34may initiate a 1D X-Y point scan of the object surface through X-Ymovement of the laser light source 33. As shown in FIG. 4, the laser 33may point scan the surface of the object 26 by projecting light spotsalong 1D horizontal scanning lines—two of which S_(R) 66 and S_(R+1) 68are identified by dotted lines in FIG. 4. Because of the curvature ofthe surface of the object 26, the light spots 70-73 may form thescanning line S_(R) 66 in FIG. 4. For ease of illustration and clarity,the light spots constituting the scan line S_(R+1) 68 are not identifiedusing reference numerals. The laser 33 may scan the object 26 along rowsR, R+1, and so on, one spot at a time—for example, in the left-to-rightdirection. The values of “R”, “R+1”, and so on, are with reference torows of pixels in the 2D pixel array 42 and, hence, these values areknown. For example, in the 2D pixel array 42 in FIG. 4, the pixel row“R” is identified using reference numeral “75” and the row “R+1” isidentified using reference numeral “76.” It is understood that rows “R”and “R+1” are selected from the plurality of rows of pixels forillustrative purpose only.

The plane containing the rows of pixels in the 2D pixel array 42 may becalled the image plane, whereas the plane containing the scanning lines,like the lines S_(R) and S_(R+1), may be called the scanning plane. Inthe embodiment of FIG. 4, the image plane and the scanning plane areoriented using epipolar geometry such that each row of pixels R, R+1,and so on, in the 2D pixel array 42 forms an epipolar line of thecorresponding scanning line S_(R), S_(R+1), and so on. A row of pixels“R” may be considered epipolar to a corresponding scanning line “S_(R)”when a projection of an illuminated spot (in the scanning line) onto theimage plane may form a distinct spot along a line that is the row “R”itself. For example, in FIG. 4, the arrow 78 illustrates theillumination of the light spot 71 by the laser 33, whereas the arrow 80shows that the light spot 71 is being imaged or projected along the row“R” 75 by the focusing lens 44. Although not shown in FIG. 4, it isobserved that all of the light spots 70-73 will be imaged bycorresponding pixels in the row “R.” Thus, in one embodiment, thephysical arrangement, such as the position and orientation, of the laser33 and the pixel array 42 may be such that illuminated light spots in ascanning line on the surface of the object 26 may be captured ordetected by pixels in a corresponding row in the pixel array 42—that rowof pixels thus forming an epipolar line of the scanning line. Althoughnot shown in FIG. 4, it is observed here that, in one embodiment, ascanning line—such as the scanning line S_(R)—may not be perfectlystraight, but may be curved or slanted. Such not-so-perfect laser scanlines may result, for example, when there is a misalignment between thelaser 33 and the pixel array 42. The misalignment may be due tolimitations on mechanical/physical tolerances of various parts assembledin the system 15 or due to any discrepancy in the arrangement or finalassembly of these parts. In case of a curved/slanted scanning line, twoor more rows of pixels (in the pixel array 42) may collectively form anepipolar line of the curved scanning line. In other words, in particularembodiments, a single row of pixels may only form a portion of theepipolar line. In any event, the teachings of the present disclosureremain applicable regardless of whether a single row or a group of rowsof pixels in the image plane forms an epipolar line of a correspondingscanning line. However, for ease of explanation and without the loss ofgenerality, the discussion below may primarily refer to theconfiguration in which a single row of pixels forms an entire epipolarline.

It is understood that the pixels in the 2D pixel array 42 may bearranged in rows and columns. An illuminated light spot may bereferenced by its corresponding row and column in the pixel array 42.For example, in FIG. 4, the light spot 71 in the scanning line S_(R) isdesignated as “X_(R,i)” to indicate that the spot 71 may be imaged byrow “R” and column “i” (C_(i)) in the pixel array 42. The column C_(i)is indicated by dotted line 82. Other illuminated spots may be similarlyidentified. As noted before, it may be possible that light reflectedfrom two or more lights spots may be received by a single pixel in arow, or, alternatively, light reflected from a single light spot may bereceived by more than one pixel in a row of pixels. The timestamp-basedapproach discussed later may remove the ambiguities in depthcalculations arising from such multiple or overlapping projections.

In the illustration of FIG. 4, the arrow having reference numeral “84”represents the depth or distance “Z” (along the Z-axis) of the lightspot 71 from the X-axis along the front of the device 15—such as theX-axis shown in FIG. 2. In FIG. 4, a dotted line having the referencenumeral “86” represents such axis, which may be visualized as beingcontained in a vertical plane that also contains the projection optics35 and the collection optics 44. However, for ease of explanation of thetriangulation method, the laser source 33 is shown in FIG. 4 as being onthe X-axis 86 instead of the projection optics 35. In atriangulation-based approach, the value of “Z” may be determined usingthe following equation:

$\begin{matrix}{Z = \frac{hd}{q - {h\; \tan \; \theta}}} & (1)\end{matrix}$

The parameters mentioned in the above equation (1) are also shown inFIG. 4. Based on the physical configuration of the device 15, the valuesfor the parameters on the right-hand side of equation (1) may bepre-determined. In equation (1), the parameter “h” is the distance(along the Z-axis) between the collection optics 44 and the image sensor(which is assumed to be in a vertical plane behind the collection optics44); the parameter “d” is the offset distance between the light source33 and the collection optics 44 associated with the image sensor 24; theparameter “q” is the offset distance between the collection optics 44and a pixel that detects the corresponding light spot—here, thedetecting/imaging pixel “i” is represented by column C_(i) associatedwith the light spot X_(R,i) 71; and the parameter “θ” is the scan angleor beam angle of the light source for the light spot underconsideration—here, the light spot 71. Alternatively, the parameter “q”may also be considered as the offset of the light spot within the fieldof view of the pixel array 42.

It is seen from equation (1) that only the parameters “θ” and “q” arevariable for a given point scan; the other parameters “h” and “d” areessentially fixed due to the physical geometry of the device 15. Becausethe row “R” 75 is at least a portion of an epipolar line of the scanningline S_(R), the depth difference or depth profile of the object 26 maybe reflected by the image shift in the horizontal direction—asrepresented by the values of the parameter “q” for different lightsspots being imaged. As discussed later below, the time-stamp basedapproach according to particular embodiments of the present disclosuremay be used to find the correspondence between the pixel location of acaptured light spot and the corresponding scan angle of the laser source33. In other words, a timestamp may represent an association between thevalues of parameters “q” and “θ”. Thus, from the known value of the scanangle “θ” and the corresponding location of the imaged light spot (asrepresented by the parameter “q”), the distance to that light spot maybe determined using the triangulation equation (1).

It is observed here that usage of triangulation for distancemeasurements is described in the relevant literature including, forexample, the United States Patent Application Publication No. US2011/0102763 to Brown et al. The discussion in the Brown publicationrelated to triangulation-based distance measurement is incorporatedherein by reference in its entirety.

FIG. 5 illustrates an exemplary time-stamping for scanned light spotsaccording to one embodiment of the present disclosure. Additionaldetails of generation of individual timestamps are provided later suchas, for example, with reference to discussion of FIG. 8. In contrast toFIG. 4, in the embodiment of FIG. 5, the collection optics 44 and thelaser 33 are shown in an offset arrangement to reflect the actualphysical geometry of these components as shown in the embodiment of FIG.2. By way of an example, the scanning line 66 is shown in FIG. 5 alongwith corresponding light spots 70-73, which, as mentioned before, may beprojected based on a left-to-right point scan of the object surface bythe sparse laser point source 33. Thus, as shown, the first light spot70 may be projected at time instant “t₁,” the second light spot 71 maybe projected at time instant “t₂,” and so on. These light spots may bedetected/imaged by respective pixels 90-93 in the pixel row “R” 75—whichis an epipolar line of the scanning line S_(R) as discussed earlier. Inone embodiment, the charge collected by each pixel when detecting alight spot may be in the form of an analog voltage, which may be outputto the image processing unit 46 for pixel-specific depth determinationas discussed below. The analog pixel outputs (pixouts) are collectivelyindicated by arrow 95 in FIG. 5.

As shown in FIG. 5, each detecting pixel 90-93 in row R may have anassociated column number—here, columns C₁ through C₄. Furthermore, it isseen from FIG. 4 that each pixel column C_(i) (i=1, 2, 3, and so on) hasan associated value for the parameter “q” in equation (1). Thus, when apixel-specific timestamp t₁-t₄ is generated for the detecting pixels90-93 (as discussed in more detail later below), the timestamp mayprovide an indication of the pixel's column number and, hence, thepixel-specific value of the parameter “q.” Additionally, in oneembodiment, the spot-by-spot detection using pixels in the pixel array42 may allow the image processing unit 46 to “link” each timestamp withthe corresponding illuminated spot and, hence, with the spot-specificscan angle “θ”—because the laser 33 may be suitably controlled toilluminate each spot in the desired sequence with pre-determined valuesfor spot-specific scan angles “θ”. Thus, timestamps providecorrespondence between the pixel location of a captured laser spot andits respective scan angle—in the form of the values for parameters “q”and “θ” in equation (1) for each pixel-specific signal received from thepixel array 42. As discussed before, the values of the scan angle andthe corresponding location of the detected spot in the pixel array 42—asreflected through the value of the parameter “q” in equation (1)—mayallow depth determination for that light spot. In this manner, the 3Ddepth map for the surface of the object 26 in the field of view of thepixel array 42 may be generated.

FIG. 6 shows exemplary circuit details of the 2D pixel array 42 and aportion of the associated processing circuits in the image processingunit 46 of the image sensor 24 in FIGS. 1-2 according to one embodimentof the present disclosure. As noted before, the pixel array 42 is shownhaving nine pixels 100-108 arranged as a 3×3 array for ease ofillustration only; in practice, a pixel array may contain hundreds ofthousands or millions of pixels in multiple rows and columns. In oneembodiment, each pixel 100-108 may have an identical configuration asshown in FIG. 6. In the embodiment of FIG. 6, the 2D pixel array 42 is aComplementary Metal Oxide Semiconductor (CMOS) array in which each pixelis a Four Transistor Pinned Photo-diode (4T PPD) pixel. For ease ofillustration, the constituent circuit elements of only pixel 108 arelabeled with reference numerals. The following discussion of theoperation of the pixel 108 equally applies to the other pixels 101-107and, hence, the operation of each individual pixel is not describedherein.

As shown, the 4T PPD pixel 108 (and similar other pixels 101-107) maycomprise a pinned photo-diode (PPD) 110 and four N-channel Metal OxideSemiconductor Field Effect Transistors (NMOS) 111-114 connected asillustrated. In some embodiments, the pixels 100-108 may be formed ofP-channel Metal Oxide Semiconductor Field Effect Transistors (PMOS) orother different types of charge transfer devices. The transistor 111 mayoperate as a Transfer Gate (TG), Floating Diffusion (FD) transistor.Broadly, the 4T PPD pixel 108 may operate as follows: First, the PPD 110may convert the incident photons into electrons, thereby converting theoptical input signal into an electrical signal in the charge domain.Then, the transfer gate 111 may be “closed” to transfer all thephoton-generated electrons from the PPD 110 to the floating diffusion.The signal in the charge domain thus gets converted to the voltagedomain for ease of subsequent processing and measurements. The voltageat the floating diffusion may be later transferred as a pixout signal toan Analog-to-Digital Converter (ADC) using the transistor 114 andconverted into an appropriate digital signal for subsequent processing.More details of the pixel output (PIXOUT) generation and processing areprovided below with reference to discussion of FIG. 8.

In the embodiment of FIG. 6, a row decoder/driver 116 in the imageprocessing unit 46 is shown to provide three different signals tocontrol the operation of the pixels in the pixel array 42 to generatethe column-specific pixout signals 117-119. In the embodiment of FIG. 5,the output 95 may collectively represent such PIXOUT signals 117-119. ARow Select (RSEL) signal may be asserted to select an appropriate row ofpixels. In one embodiment, the row to be selected is the epipolar lineof the current scanning line (of light spots) being projected by thelaser source 33. The row decoder/driver 116 may receive the address orcontrol information for the row to be selected via the rowaddress/control inputs 126, for example, from the processor 19. In thepresent discussion, it is assumed that the row decoder/driver 116selects the row of pixels containing the pixel 108. A transistor, suchas the transistor 114, in each row of pixels in the pixel array 42 maybe connected to a respective RSEL line 122-124 as shown. A Reset (RST)signal may be applied to pixels in the selected row to reset thosepixels to a pre-determined high voltage level. Each row-specific RSTsignal 128-130 is shown in FIG. 6 and explained in more detail withreference to the waveforms in FIG. 8. A transistor, such as thetransistor 112, in each pixel may receive the respective RST signal asshown. A Transfer (TX) signal may be asserted to initiate transfer ofthe pixel-specific output voltage (PIXOUT) for subsequent processing.Each row-specific TX line 132-134 is shown in FIG. 6. A transfer-gatetransistor, such as the transistor 111, may receive the respective TXsignal as illustrated in FIG. 6.

As mentioned before, in particular embodiments of the presentdisclosure, the 2D array 42 and the rest of the rest of the componentsin the image sensor unit 24 may be used for 2D RGB (or non-RGB) imagingas well as for 3D depth measurements. Consequently, as shown in FIG. 6,the image sensor unit 24 may include a pixel column unit 138 thatincludes circuits for Correlated Double Sampling (CDS) as well ascolumn-specific ADCs—one ADC per column of pixels—to be used during 2Dand 3D imaging. The pixel column unit 138 may receive the PIXOUT signals117-119 and process them to generate a digital data output (Dout) signal140 from which 2D image may be generated or 3D depth measurements can beobtained. The pixel column unit 138 may also receive a reference input142 and a ramp input 143 during processing of the PIXOUT signals117-119. More details of the operation of the unit 138 are providedlater below. In the embodiment of FIG. 6, a column decoder unit 145 isshown coupled to the pixel column unit 138. The column decoder 145 mayreceive a column address/control input 147, for example, from theprocessor 19, for the column to be selected in conjunction with a givenrow select (RSEL) signal. The column selection may be sequential,thereby allowing sequential reception of the pixel output from eachpixel in the row selected by the corresponding RSEL signal. Theprocessor 19 may be aware of the currently-projected scanning line oflight spots and, hence, may provide appropriate row address inputs toselect the row of pixels that forms the epipolar line of the currentscanning line and may also provide appropriate column address inputs toenable the pixel column unit 138 to receive outputs from the individualpixels in the selected row.

It is observed here that although the discussion herein primarilyfocuses on the 4T PPD pixel design shown in FIG. 6 for 2D and 3D imagingaccording to teachings of the present disclosure, different types ofpixels may be used in the pixel array 42 in other embodiments. Forexample, in one embodiment, each pixel in the pixel array 42 may be a 3Tpixel, which omits the transfer gate transistor—like the transistor 111in the 4T PPD design in FIG. 6. In other embodiments, 1T pixels or 2Tpixels may be used as well. In yet another embodiment, each pixel in thepixel array 42 may have a shared-transistor pixel configuration, wheretransistors and read-out circuitry can be shared among two or moreneighboring pixels. In the shared-transistor pixel configuration, eachpixel may have at least one photo-diode and one transfer-gatetransistor; the rest of the transistors can be shared among two or morepixels. One example of such a shared-transistor pixel is the 2-shared(1×2) 2.5T pixel where five transistors (T) are used for two pixels,resulting in a 2.5T/pixel configuration. Another example of ashared-transistor pixel that may be used in the pixel array 42 is the1×4 4-shared pixel, in which 4 pixels share the readout circuitry, buteach one has at least one photo-diode and one TX (transfer-gate)transistor. Other pixel configurations than those listed here may besuitably implemented for 2D and 3D imaging as per teachings of thepresent disclosure.

FIG. 7A is an exemplary layout of an image sensor unit, such as theimage sensor unit 24 in FIG. 6, according to one embodiment of thepresent disclosure. For the sake of brevity, only a brief discussion ofthe architecture in FIG. 7A is provided herein; more relevantoperational details are provided later with reference to FIGS. 8 and10-11. In the embodiment of FIG. 7A, various component blocks other thanthe 2D pixel array 42 may form a part of the pixel control unit 46 inFIG. 2. As shown, the image sensor unit 24 in FIG. 7A may include a rowdecoder unit 149 and a row driver unit 150, both of which collectivelycomprise the row decoder/driver 116 in FIG. 6. Although not shown inFIG. 7A, the row decoder unit 149 may receive a row address input (likethe input 126 shown in FIG. 6), for example, from the processor 19, anddecode the input to enable the row driver unit 150 to provideappropriate RSEL, RST, and TX signals to the row selected/decoded by therow decoder 149. The row driver unit 150 may also receive controlsignals (not shown), for example, from the processor 19, to configurethe row driver 150 to apply appropriate voltage levels for the RSEL,RST, and TX signals. In the image sensor 24 in FIG. 7A, a column ADCunit 153 may represent the pixel column unit 138 in FIG. 6. For ease ofillustration, in FIG. 7A, various row-specific driver signals—such asthe RSEL, RST, and TX signals—from the row driver 150 are collectivelyreferenced using a single reference numeral “155.” Similarly, allcolumn-specific pixel outputs (pixouts)—like the pixouts 117-119 in FIG.6—are collectively referenced using a single reference numeral “157.”The column ADC unit 153 may receive the pixout signals 157 and thereference input 142 (from a reference signal generator 159) and the rampsignal 143 to generate a pixel-specific output by the correspondingcolumn-specific ADC for the pixel's column. The 3D imaging is discussedin more detail later with reference to FIG. 8. In one embodiment, theADC unit 153 may include circuitry for CDS—as in case of the pixelcolumn unit 138 in FIG. 6—to generate a CDS output (not shown) that isthe difference between the pixel's reset level and the received signallevel. In particular embodiments, the 3D depth values may be combinedwith the 2D image to generate a 3D image of the object.

The column ADC unit 153 may include a separate ADC per pixel column inthe 2D array 42. Each column-specific ADC may receive a respective rampinput 143 (from a ramp signal generator 163) along with the pixoutsignals 157. In one embodiment, the ramp signal generator 163 maygenerate the ramp input 143 based on the reference voltage levelreceived from the reference signal generator 159. Each column-specificADC in the ADC unit 153 may process the received inputs to generate thecorresponding digital data output (Dout) signal 140. From the columndecoder 145, the ADC unit 153 may receive information about which columnADC output to be readout and sent to the Dout bus 140, and may alsoreceive information about which column to select for a given row toreceive the appropriate pixel output. Although not shown in FIG. 7A, thecolumn decoder unit 145 may receive a column address input (like theinput 147 in FIG. 6), for example, from the processor 19, and decode theinput to enable the column ADC unit 153 to select the appropriate pixelcolumn. In the embodiment of FIG. 7A, the decoded column address signalsare collectively identified using the reference numeral “165.”

The digital data outputs 140 from the ADC units may be processed by adigital processing block 167. In one embodiment, for the 2D RGB imagingmode, each ADC-specific data output 140 may be a multi-bit digital valuethat substantially corresponds to the actual photon charge collected bythe respective pixel. On the other hand, in the 3D depth measurementmode, each ADC-specific data output 140 may be a timestamp valuerepresenting the time instant when the respective pixel detects itscorresponding light spot. This timestamping approach according to theteachings of the present disclosure is discussed later in more detail.The digital processing block 167 may include circuits to provide timinggeneration; Image Signal Processing (ISP) such as, for example,processing of data outputs 140 for the 2D imaging mode; depthcalculations for the 3D imaging mode; and so on. In that regard, thedigital processing block 167 may be coupled to an interface unit 168 toprovide the processed data as an output 170, for example, to enable theprocessor 19 to render a 2D RGB/non-RGB image or a 3D depth image of the3D object 26 on a display screen (not shown) of the device 15. Theinterface unit 168 may include a Phase-Locked Loop (PLL) unit forgeneration of clock signals that support the timing generationfunctionality in the digital processing block 167. Furthermore, theinterface unit 168 may also include a Mobile Industry ProcessorInterface (MIPI) that provides an industry-standard hardware andsoftware interface to other components or circuit elements in the device15 for the data generated by the digital block 167. The MIPIspecifications support a broad range of mobile products and providespecifications for a mobile device's camera, display screen, powermanagement, battery interface, and the like. The MIPI-standardizedinterfaces may yield an improved operability between a mobile device'speripherals—such as a smartphone's camera or display screen—and themobile device's application processor(s), which may not be from the samevendor as the vendor (or vendors) providing the peripherals.

In the embodiment of FIG. 7A, a timestamp calibration unit 171 is showncoupled to the column ADC unit 153 to provide appropriate calibrationsignals 172 to individual column-specific ADCs to enable eachcolumn-specific ADC unit to generate an output representing apixel-specific timestamp value in the 3D measurement mode. Although notshown in FIG. 7A, it is understood that, in particular embodiments, thecalibration unit 171 may be coupled to the digital block 167 as well fortimestamp calibration related processing support. The timestampingapproach and related calibration aspects are discussed in more detailwith reference to FIGS. 8-11.

FIG. 7B shows architectural details of an exemplary CDS+ADC unit 175 for3D depth measurement according to one embodiment of the presentdisclosure. For ease of discussion, the unit 175 may be referred belowto as “ADC unit,” however, it is understood that the unit 175 may alsoinclude CDS functionality in addition to the ADC functionality. In FIG.7B, the capacitor 176 represents a simplified version of a CDS unit. Inone embodiment, each column of pixels in the 2D pixel array 42 may havea column-specific, single slope ADC unit similar to the ADC unit 175. Inother words, each pixel in a given column may share the same ADC unit,like the ADC unit 175. Thus, in the embodiment of FIG. 6, there may bethree ADC units in the pixel column unit 138—one ADC per column. Inparticular embodiments, the column-specific ADC units 175 may be part ofthe column ADC unit 153 in FIG. 7A. As shown, the ADC 175 in theembodiment of FIG. 7B may include two Operational TransconductanceAmplifiers (OTA) 177, 179, connected in series with a binary counter 181and a line memory unit 183. For ease of illustration, only the inverting(−) and non-inverting (+) voltage inputs to the OTAs 177, 179 are shownin FIG. 7B; the biasing inputs and the power supply connections are notshown. It is understood that an OTA is an amplifier whose differentialinput voltage produces an output current. Thus, an OTA may be consideredas a voltage-controlled current source. The biasing inputs may be usedto provide currents or voltages to control the amplifier'stransconductance. The first OTA 177 may receive—from the CDS unit 176—aCDS version of the pixout voltage from a pixel, such as the pixel 108 inFIG. 6, that is selected in the activated row using the column numberreceived from the column decoder 145. The CDS version of a pixout signalmay be referred to as a “PIX_CDS” signal. The OTA 177 may also receive aVramp voltage 143 from the ramp signal generator 163 (FIG. 7A). The OTA177 may generate an output current when the pixout voltage 157 dropsbelow the Vramp voltage 143, as discussed below with reference to FIG.8. The output of the OTA 177 may be filtered by the second OTA 179before being applied to the binary counter 181. In one embodiment, thebinary counter 181 may be a 10-bit ripple counter that receives a Clock(Clk) input 185 and generates a timestamp value 186 based on the clockcycles counted during a pre-determined time triggered by the generationof the output current by the first OTA 177. In the context of theembodiment in FIG. 7B, the Clk input 185 may be a system-wide clock oran image sensor-specific clock generated by the PLL unit 168 or otherclock generator (not shown) in the device 15. The pixel-specifictimestamp value 186 may be stored in the line memory 183 against thecolumn number (column #) of the pixel, and subsequently output to thedigital processing block 167 as the Dout signal 140. The column numberinput 165 may be received from the column decoder unit 145 shown in FIG.7A.

In particular embodiments, the RGB color model may be used for sensing,representation, and display of images on mobile devices such as, forexample, the device 15 in FIGS. 1-2. In the RGB color model, the lightsignals having three primary colors—red, green, and blue—may be addedtogether in various ways to produce a broad array of colors in the finalimage. The CDS method may be used in 2D RGB imaging to measure anelectrical value, such as a pixel/sensor output voltage, in a mannerthat allows removal of an undesired offset. For example, a CDS unit,like the CDS unit 176, may be employed in each column-specific ADC unit,like the ADC unit 175, to perform correlated double sampling. In CDS,the output of the pixel may be measured twice—once in a known condition,and once in an unknown condition. The value measured from the knowncondition may be then subtracted from the value measured from theunknown condition to generate a value with a known relation to thephysical quantity being measured—here, the photoelectron chargerepresenting the pixel-specific portion of the image signal. Using CDS,noise may be reduced by removing the reference voltage of the pixel(such as, for example, the pixel's voltage after it is reset) from thesignal voltage of the pixel at the end of each integration period. Thus,in CDS, before the charge of a pixel is transferred as an output, thereset value is sampled. The reference value is “deducted” from the valueafter the charge of the pixel is transferred.

It is observed here that, in particular embodiments, the ADC unit 175may be used for both—2D imaging as well as 3D depth measurements. Allthe inputs for such shared configuration, however, are not shown in FIG.7B. In the shared use case, the corresponding Vramp signal may bedifferent as well for 2D imaging.

FIG. 8 is a timing diagram 190 that shows exemplary timing of differentsignals in the system 15 of FIGS. 1-2 to generate timestamp-basedpixel-specific outputs in a 3D mode of operation according to particularembodiments of the present disclosure. As noted before, in particularembodiments, all pixels in the same image sensor 24 may be used for 2Das well as 3D imaging.

Briefly, as discussed earlier with reference to FIGS. 4-5, the 3D object26 may be point-scanned—one spot at a time—by the laser light source 33along a row “R” 75 of the pixel array 42, where “R” is known based onits epipolar relation with the scanning line S_(R) 66. After scanningone row, the scanning operation repeats with another row. When the laserprojects the next spot, the earlier-projected light spot may be imagedby the corresponding pixel in the row R. The pixel-specific outputs fromall the pixels in the row R may be read out to the depth processingcircuit/module in the digital processing block 167 (FIG. 7A).

To generate a pixel-specific output, the corresponding row may have tobe initially selected using an RSEL signal. In the context of FIG. 8, itis assumed that the row decoder/driver 116 in FIG. 6 selects the row ofpixels containing pixels 106-108 by asserting the RSEL signal 122 to a“high” level as shown in FIG. 8. Thus, all the pixels 106-108 areselected together. For ease of discussion, the same reference numeralsare used in FIG. 8 for the signals, inputs, or outputs that are alsoshown in FIGS. 6-7. Initially, all the pixels 106-108 in the selectedrow may be reset to a high voltage using the RST line 128. The “reset”level of a pixel may represent an absence of the pixel-specificdetection of a corresponding light spot. In the 3D mode according to oneembodiment of the present disclosure, the RST signal 128 may be releasedfrom its high level for a pre-determined time to facilitate integrationof photoelectrons received by the pixels 106-108, so as to obtain thecorresponding pixel output (pixout) signals 117-119—two of which areshown in FIG. 8 and discussed later below. The PIXOUT1 signal 119represents the output supplied to a corresponding ADC unit by the pixel108, and is shown using a dashed line having the pattern “— ⋅⋅ — ⋅⋅ —”.The PIXOUT2 signal 118 represents the output supplied to a correspondingADC unit by the pixel 107, and is shown using a dashed line having thepattern “⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅”. It is noted here that, in one embodiment,other RST lines—like the lines 129-130 in FIG. 6—may remain high or “on”for unselected rows to prevent blooming. It is noted here that, strictlyspeaking, the PIXOUT signals 118-119 in FIG. 8 (and similar pixoutsignals in FIG. 11) may be slightly modified by a CDS unit—such as, forexample, the CDS unit 176 in FIG. 7B—before being applied as PIX_CDSsignals to the first OTA—like the OTA 177 in FIG. 7B—in a respectivecolumn-specific ADC unit, such as the ADC unit 175 in FIG. 7B. However,for the simplicity of illustration and ease of discussion, the PIXOUTsignals in FIGS. 8 and 11 are treated as representatives of respectivePIX_CDS signals (not shown) and are considered as having been directly“input” to the respective OTAs 177.

After reset, when a photodiode in a pixel receives incidentluminance—such as, for example, the photoelectrons in the lightreflected from a light spot projected on the surface of the 3D object26, the photodiode may generate corresponding photocurrent. A pixel'sdetection of incident light may be called an “ON event,” whereas adecrease in the intensity of incident light may produce an “OFF event.”The photocurrent generated in response to an ON event may decrease thepixel output voltage (PIXOUT) from its initial reset level. A pixel thusfunctions as a transducer to convert received luminance/light signalinto a corresponding electrical (analog) voltage, which is generallydesignated as a PIXOUT signal in FIGS. 6-8 and 10-11. Each pixel may beread individually and, preferably, in the sequence in which thecorresponding light spots are projected by the laser source. The analogpixout signal may be converted to a digital value by the correspondingcolumn ADC. In the 2D imaging mode, the ADC may function as ananalog-to-digital converter and generate a multi-bit output. However, asdiscussed below, in the 3D depth measurement mode, the ADC may functionas a time-to-digital converter (TDC) and generate a timestamp valuerepresenting the time when a light spot is detected by a pixel.

Referring again to FIG. 8, after the pixel reset is done (with RST 128high), the column ADCs associated with pixels 106-108 may be reset aswell before the RST is released. However, the transfer (TX) signal 132may remain high throughout. The ADCs may be reset using either a commonADC reset signal or individual ADC-specific reset signals. In theembodiment of FIG. 8, a common ADC_RST signal 192 is shown to have beenbriefly asserted (to a high level) to reset the column-specificADCs—like the ADC 175—in the column ADC unit 153 (FIG. 7A). In oneembodiment, the ADCs may be reset to a pre-determined binary value—suchas a binary “0” or other known number—after the pixels are reset. InFIG. 8, these reset values for ADCs associated with pixels 108 and 107are shown by “fields” 194-195 in the signals ADCOUT1 (or ADC output “A”)and ADCOUT2 (or ADC output “B”), respectively. It is noted here that theterm “field” is used here for the sake of convenience only whendiscussing the ADC outputs shown in FIG. 8. It is understood that an ADCoutput may not actually consist of all of such “fields” at the sametime, but may be a specific digital value depending on the ADC's currentstage of signal processing—if the ADC is reset, its output may be abinary “0”; if the ADC is triggered to count clock pulses, its outputmay be a count value as in case of the 3D depth measurements in FIG. 8;or if the ADC is used for 2D color imaging, then its output may be amulti-bit value representing an image signal. Thus, the ADC outputsignals in FIG. 8 are depicted with such “fields” merely to illustratedifferent digital values an ADC may internally generate in progressingtoward the final output. In FIG. 8, the reference numeral “197” is usedto refer to the ADCOUT1 signal representing the output of the ADCassociated with the pixel 108, and the reference numeral “198” is usedto refer to the ADCOUT2 signal representing the output of the ADCassociated with the pixel 107. Each of the outputs 197-198 may appear asthe Dout signal 140 (FIGS. 6-7) when the respective ADC is selected bythe column decoder during memory readout. Prior to being reset, the ADCoutputs 197-198 may have unknown values, as indicated by the notation“x” in the fields 199-200.

After ADCs are reset, a pre-determined threshold value may be enabled byde-asserting the ramp input (Vramp) 143 to a pre-defined voltage levelafter the pixel reset signal 128 and ADC reset signal 192 are released.In the embodiment of FIG. 8, the RAMP input 143 is common to allcolumn-specific ADCs, thereby providing the same Vramp voltage to eachADC. However, in other embodiments, different Vramp values may beapplied to two or more ADCs through separate, ADC-specific ramp inputs.Furthermore, in particular embodiments, the Vramp threshold may be aprogrammable parameter, allowing it to be variable as desired. After thethreshold (RAMP signal) is enabled, the pixel-specific ADCs may wait forthe corresponding pixel's “ON event” before starting their binarycounters—like the counter 181 in FIG. 7B.

In the 3D depth measurement mode, each ADC may generate a single bitoutput (representing binary “0” or “1”), as opposed to a multi-bitoutput in case of the 2D imaging mode. Thus, in case of an RGB sensor,any color information received by a pixel in the RGB pixel array 42 maybe effectively ignored in the 3D mode. In the absence of any incidentlight detected by a pixel, the corresponding ADCOUT signal may remain atthe binary “0” value. Thus, columns without any ON events may continueto have digital value “0” (or other known number) for their respectiveADCOUT signals. However, as noted before, when a pixel is hit withincident light, its PIXOUT line may start to droop from its resetlevel—as indicated by the downward slopes of the PIXOUT1 and PIXOUT2signals in FIG. 8. Assuming that pixel charge is read starting with thepixel that receives the charge first, such a reading may start with theright-most pixel in a row and end with the left-most pixel as shown, forexample, in FIG. 5, where “t₁” is the earliest time instant and “t₄” isthe latest one. Thus, in the embodiment of FIG. 8, the output of thepixel 108 (PIXOUT1) may be read before that of the pixel 107 (PIXOUT2).As soon as the progressively-drooping PIXOUT1 reaches the Vrampthreshold 143, the single-bit ADCOUT1 may flip from binary “0” to binary“1.” However, instead of outputting the bit “1,” the corresponding ADCmay record the time when the bit flips (from “0” to “1”). In otherwords, the ADC associated with the pixel 108 may function as atime-to-digital converter, by starting the binary counter in the ADC, asindicated by the “up count” field 202 in ADCOUT1. During the “up count”period, the counter in the ADC may count the clock pulses in the CLKsignal 185, which may be applied to each ADC as shown, for example, inFIG. 7B. The counted clock pulses are shown by the Counter Clock-1signal 204 in FIG. 8, and the counted value (in the “up count” field)may be provided as a pixel-specific output for the pixel 108. A similarcounting may occur at the ADC associated with pixel 107 for the chargecollected by the pixel 107, as indicated by the Counter Clock-2 signal205 in FIG. 8. The pixel-specific counted value (in the “up count” field207) may be provided by the respective ADC as a pixel-specific outputfor the pixel 107. After scanning all pixels in one row, thepixel-by-pixel charge collection operation may repeat with another row,while the outputs from the earlier-scanned row are read out to the depthcalculation unit in the digital block 167.

Each ADC output may effectively represent a respective “timestamp” valueproviding a temporal indication of a pixel's detection of a light spoton the object surface illuminated by the laser light source 33. A“timestamp” may be considered to capture the light arrival time for apixel. In one embodiment, a timestamp value may be generated for adetected light spot by the digital processing block 167 from the countvalue (of the counted clock pulses) received from an ADC unit. Forexample, the digital block 167 may generate a timestamp by relating thecount value to an internal system time or other reference time. Thetimestamp is generated at the receiving end and, hence, may notnecessarily represent the exact time when the corresponding light spotwas projected by the light source. However, the timestamp values mayallow the digital block 167 to establish a temporal correlation amongtime-stamped light spots, thereby allowing the digital block 167 todetermine distances to time-stamped light spots in the time-wise orderspecified by the temporal correlation—the distance to the earliestilluminated light spot being determined first, and so on, until thedistance to the last-illuminated light spot is determined. In oneembodiment, the timestamping approach may also facilitate resolution ofthe ambiguity that may arise from multiple light spots being imaged onthe same pixel, as discussed later.

All ADC-based counters may stop simultaneously such as, for example,when the ramp signal 143 is asserted again after a pre-determined timeperiod has elapsed. In FIG. 8, the transition of the ramp signal 143,marking the conclusion of the pre-determined time period for pixelcharge integration, is indicated by dotted line 210. The RSEL 122 andthe RST 128 signals may also transition their states substantiallysimultaneously with the change in the level of the ramp signal 143 (atline 210). It is observed here that, in one embodiment, all ADC-basedcounters may be reset at line 210. In another embodiment, all ADC-basedcounters may be reset at any time prior to the selection of the next rowof pixels for reading the pixel charge. Despite resetting of ADCcounters upon conclusion of scanning of pixels in one row, the timestampvalue for each pixel in the pixel array 42 may remain distinct becauseof the relational establishment of the timestamp value against aninternal system time or other reference source of time, which may remainglobal and continuously-running.

It is observed here that, in the embodiment of FIG. 8, a later-scannedpixel—such as the pixel 107—may have a smaller ADC output than the pixelthat is scanned earlier—such as the pixel 108. Thus, as shown, theADCOUT2 may have less count value (or less number of clock pulsescounted) than the ADCOUT1. Alternatively, in another embodiment, alater-scanned pixel may have a larger ADC output than an earlier-scannedpixel, for example, when each ADC-specific counter starts counting whena pixel is reset and stops counting when an “ON event” is detected—suchas, for example, when the pixel's pixout signal droops below a giventhreshold (Vramp).

It is noted here that circuits and waveforms shown in FIGS. 6-8 and 11are based on single-slope ADCs with per column up-counters. However, itis understood that the time-stamping approach may be implemented withup- or down-counters depending on the design choice. Furthermore, singleslope ADCs with global counters may be used as well. For example, in oneembodiment, instead of using individual, column-based counters, a globalcounter (not shown) may be shared by all column ADCs. In that case, theADCs may be configured such that the column memory—like the line memory183 in FIG. 7B—in each ADC may latch the output of the global counter togenerate an appropriate ADC-specific output when a column-basedcomparator unit (not shown) detects an “ON event” such as, for example,when it first senses the respective pixout signal drooping below theramp threshold 143.

It is observed here that, when a row of light spots is scanned along thesurface of the object, two or more different spots from the objectscanned may be imaged on the same pixel. The spots may be in the samescanning line or may be on adjacent scanning lines. When multiple spotsare scanned across the surface of the object, such overlapping imagingmay negatively affect the correlation of the spots and the pixel ONevents and, hence, may cause ambiguity in the depth measurements. Forexample, it is seen from the earlier-mentioned equation (1) that thedepth measurement is related to the scan angle (θ) and the pixellocation of the imaged light spot—as given by the parameter “q” inequation (1). Thus, if the scan angle is not correctly known for a givenlight spot, the depth calculation may be inaccurate. Similarly, if twoor more light spots have the same value of “q”, the depth calculationsmay become ambiguous as well. The time-stamp based approach according toparticular embodiments of the present disclosure may be used to maintainthe correct correlation between the pixel location of a captured lightspot and the corresponding scan angle of the laser source. In otherwords, a timestamp may represent an association between the values ofparameters “q” and “θ”. Thus, if two spots land on the same pixel orcolumn (from the data output point of view), the time-to-digitalconversion in the timestamping approach may allow the imagingsystem—here, the digital processing block 167 (FIG. 7B)—to establish atemporal correlation between these two spots to identify which lightspot was received first in time. Such correlation may not be easilypossible in systems that do not use timestamping, such as, for example,the earlier-discussed stereo vision systems or the systems using thestructured light approach. As a result, such systems may need to performa lot of data searching and pixel-matching to solve the correspondenceproblem.

In one embodiment, when multiple light spots are imaged by the samepixel, timestamps of these light spots may be compared to identify theearliest-received light spot and the distance may be calculated for thatlight spot only, while ignoring all subsequently-received light spots atthe same pixel. Thus, in this embodiment, the timestamp of theearliest-received light spot may be treated as the pixel-specific outputfor the corresponding pixel. Alternatively, in another embodiment, thedistance may be calculated for the light spot that is received the lastin time, while ignoring all other light spots imaged by the same pixel.In either case, any light spot received between the first or the lastlight spot may be ignored for depth calculations. Mathematically, thescan times of light spots projected by a light source may be given ast(0), t(1), . . . , t(n), where t(i+1)−t(i)=d(t) (constant). Thepixel/column outputs may be given as a(0), a(1), . . . , a(n), which aretimestamps for the ON events and a(i) is always after t(i), but beforea(i+1). If a(i) and a(k) (i≠k) happen to be associated with the samepixel/column, only one of them may be saved as discussed before toremove any ambiguity in depth calculations. Based on the timerelationship between the scan time and the output time (represented bytimestamps), the processing unit, such as the digital block 167, canfigure out which output point(s) is missing. Although the processingunit may not be able to recover the missing location, the depthcalculations from the available output points may suffice to provide anacceptable 3D depth profile of the object. It is noted here that, in oneembodiment, it also may be possible for two different pixels to image arespective portion of the same light spot. In that embodiment, based onthe closeness of the values of the timestamp outputs from these twopixels, the processing unit may infer that a single light spot may havebeen imaged by two different pixels. To resolve any ambiguity, theprocessing unit may use the timestamps to find an “average” of therespective location values “q”, and use that average value of “q” inequation (1) to calculate the 3D depth for such “shared” light spot.

It is observed from the foregoing discussion that the timestamp-based 3Ddepth measurement using triangulation according to particularembodiments of the present disclosure allows an ADC to be operated as abinary comparator with a low resolution of just a single bit, therebyconsuming significantly less switching power in the ADC and, hence,conserving the system power. A high bit resolution ADC in traditional 3Dsensors, on the other hand, may require more processing power.Furthermore, timestamp-based ambiguity resolution may also save systempower in comparison with traditional imaging approaches that requiresignificant processing power to search and match pixel data to resolveambiguities. The latency is reduced as well because all depthmeasurements may be performed in one pass due to imaging/detection ofall point-scanned light spots in a single imaging step. In particularembodiments, each pixel in the pixel array may be a single storage pixeland, hence, can be made as small as 1 micrometer (μm) in size. In asingle storage pixel design, there may be only one photodiode and onejunction capacitor per pixel (like the transistor 111 in FIG. 6) tointegrate and store photoelectrons. On the other hand, a pixel that hasone photodiode with multiple capacitors—to store photoelectrons comingat different times—may not be reduced to such a small size. Thus, thelow-power 3D imaging system with small sensors as per particularembodiments of the present disclosure may facilitate its easyimplementation in mobile applications such as, for example, in camerasin smartphones or tablets.

As mentioned before, the same image sensor—such as the image sensor unit24 in FIGS. 1-2—may be used for both 2D imaging and 3D depthmeasurements according to one embodiment of the present disclosure. Suchdual-mode image sensor may be, for example, part of a camera system on amobile phone, smartphone, laptop computer, or tablet, or as part of acamera system in an industrial robot or VR equipment. In particularembodiments, there may be a mode switch on the device to allow a user toselect between the traditional 2D camera mode or the 3D imaging modeusing depth measurements as discussed before. In the traditional 2Dcamera mode, in particular embodiments, the user may capture color (RGB)images or snapshots of a scene or a particular 3D object within thescene. However, in the 3D mode, the user may be able to generate a 3Dimage of the object based on the camera system performing the pointscan-based depth measurements in the manner discussed earlier. In eithermodes, the same image sensor may be used in its entirety to carry outthe desired imaging. In other words, each pixel in the image sensor maybe used for either application-2D or 3D imaging.

As also mentioned before, a timestamp may be used to find correspondencebetween the following two parameters given in equation (1): the pixellocation (q) of an imaged light spot, and the corresponding scan angle(θ) of the laser source. Thus, a jitter or error in a timestamp mayreduce the minimum resolvable time (or time resolution) of thetimestamp. This, in turn, may reduce the number of laser spots that canbe scanned within one row time, which may result in the reduction of theframe rate and spatial resolution of the depth image. For example, pixelcharge collection and subsequent processing may reduce the speed withwhich timestamps may be generated. In particular embodiments, thetimestamp generation may take several microseconds, whereas laserscanning may be performed significantly faster—for example, innanoseconds. Thus, an error or jitter in one or more timestamps mayresult in further processing delays, and may also require a slower laserscan to accommodate the time needed to rectify timestamp errors or togenerate error-free timestamps. Assuming that the same time is allottedto scan each row on the 3D object, the requirement to reduce the speedof the laser scan may result in a reduced number of laser spots that canbe scanned along one row of pixels in the given scan duration. This may,in turn, reduce the spatial resolution of the 3D depth image of theobject.

Timestamp jitter and error may be caused by variations in thepropagation delays of different column-specific readout chains. The ADCunit 175 in FIG. 7B illustrates an exemplary readout chain. In certainembodiments, the line memory block 183 (FIG. 7B) may not be considered apart of the column-specific “readout chain”. However, in the discussionherein, the term “column-specific readout chain” is used to refer to thecomplete ADC configuration identified using the reference numeral “175”in FIG. 7B. As discussed before, in particular embodiments, each columnof pixels in the 2D pixel array 42 may be associated with acorresponding column-specific readout chain, like the ADC unit 175. Dueto circuit layout of components, lengths of different interconnects,device mismatches, process variations, and the realistic possibilitythat two identically-formed semiconductor circuit components may notnecessarily behave identically during operation, each column-specificreadout chain may encounter a variation in the respective propagationdelay. In other words, because propagation delay may vary from column tocolumn, each column of pixels in the pixel array 42 may encounter aslightly different propagation delay during pixel charge collection andprocessing—from sensing of the light by a pixel's photodiode to eventualoutput of a timestamp value. In particular embodiments, the term“propagation delay” may refer to the time between sensing of apixel-specific detection of luminance (by the pixel's photodiode) andwhen the corresponding PIXOUT signal reaches a pre-defined threshold.The pixel-specific detection of luminance may be “sensed” uponactivation of the pixel—such as, for example, when the row of pixels isselected by the RSEL signal (for example, the RSEL signal 122 in FIG.8). The activation of the pixel may result in the receipt of thecorresponding PIXOUT signal at an input of the pixel's column-specificreadout chain, like the pixout input 157 in the readout chain 175. Inparticular embodiments, the “pre-defined threshold” may be the assertedvalue of the Vramp input, such as the Vramp signal 143 in FIGS. 7B and8. As mentioned before, an “ON event” may be registered when aprogressively-drooping PIXOUT signal reaches the Vramp threshold. In oneembodiment, the timestamp generation may commence upon occurrence ofthis ON event as shown, for example, in the embodiment of FIG. 8. Inanother embodiment, however, the timestamp generation may conclude uponoccurrence of this ON event as, for example, in case of theearlier-mentioned global counter based counting approach. Alternatively,a column-based counter, such as the counter 181 in FIG. 7B, may beconfigured to start counting when the corresponding pixel is activatedand stop the counting when the pixout signal reaches the Vrampthreshold.

When the propagation delays of column-specific readout chains are notuniform, such delay variations may cause timestamp errors and mismatch.For example, the differences in column-specific propagation delays mayresult in a later-scanned light spot having a larger timestamp valuethan an earlier-scanned light spot if the column-specific propagationdelay for the later-scanned light spot is shorter than that for theearlier-scanned one. In that case, the timestamp-based ordering of thelight spots for image reconstruction may be in error. Therefore, it maybe desirable in certain embodiments to provide timestamp calibration toremedy the timestamp jitters/errors caused by mismatches in propagationdelays of individual column-specific readout chains. The embodiments inFIGS. 9-11 provide examples of how such timestamp calibration may becarried out without increasing the bandwidth or power consumption of thecolumn readout circuits.

FIG. 9 shows an exemplary flowchart 215 illustrating how timestampvalues may be corrected during 3D depth measurements according to oneembodiment of the present disclosure. As in the embodiment of FIG. 3,various steps illustrated in FIG. 9 may be performed by a single moduleor a combination of modules or system components in the system 15. Inthe discussion herein, by way of an example only, specific tasks aredescribed as being performed by specific modules or system components.Other modules or system components, however, may be suitably configuredto perform such tasks as well.

In FIG. 9, the operation at block 217 is similar to that at block 52 inFIG. 3. In other words, at block 217 in FIG. 9, the system 15 (morespecifically, the processor 19) may perform a 1D point scan of a 3Dobject, such as the object 26 in FIG. 2, along a scanning line using alight source, such as the light source module 22 in FIG. 2. As part ofthe point scan, the light source module 22 may be configured, forexample, by the processor 19, to project a sequence of light spots on asurface of the 3D object 26 in a line-by-line manner. At block 219, thepixel processing unit 46 in the system 15 may select a row of pixels inan image sensor, such as the 2D pixel array 42 in FIG. 2. The imagesensor 42 has a plurality of pixels arranged in a 2D array forming animage plane, and, in one embodiment, the selected row of pixels forms atleast a portion of an epipolar line of the scanning line (at block 217)on the image plane. At block 221, for a pixel in the selected row ofpixels, the pixel processing unit 46 may be operatively configured bythe processor 19 to sense a pixel-specific detection of a correspondinglight spot in the sequence of light spots. As mentioned before, in oneembodiment, such “sensing” may refer to activation of the pixel forcollection of the charge generated by the sensor's photodiode when thephotodiode detects luminance received from the corresponding light spot.The pixel-specific PIXOUT signal may represent such pixel-specificcharge generated in response to received luminance. In response tosensing the pixel-specific detection of the corresponding light spot atblock 221, the pixel processing unit 46—as suitably configured by theprocessor 19—may generate a timestamp value for the corresponding lightspot, as noted at block 223. The timestamp generation aspect is alreadydiscussed before, primarily with reference to FIGS. 7B and 8. At block225, for a column in the 2D array 42 associated with the pixel in theselected row of pixels (blocks 219, 221), the image processing unit 46may apply a column-specific correction value to the timestamp value(generated at block 223) to obtain a corrected timestamp value. As notedat block 225, in one embodiment, the column-specific correction valuerepresents a column-specific propagation delay between sensing of thepixel-specific detection (block 221) and when a pixel-specific output(PIXOUT) of the pixel in the selected row of pixels reaches apre-defined threshold (such as the Vramp threshold). Such correctedtimestamp value may be obtained for the output of each pixel in theselected row of pixels to remedy the timestamp jitters/errors caused bymismatches in propagation delays of individual column-specific readoutchains. Consequently, at block 227, the image processing unit 46 maydetermine the 3D distance (or depth) to the corresponding light spot onthe surface of the 3D object based at least on the corrected timestampvalue (at block 225) and on a scan angle used by the light source forprojecting the corresponding light spot (at block 217). As noted before,a timestamp may provide the needed correspondence between the pixellocation (q) of an imaged light spot and the corresponding scan angle(θ) of the laser source. These and other parameters used in equation (1)for a triangulation-based depth measurement are illustrated in FIG. 4.

FIG. 10 is an exemplary layout 230 of a timestamp calibration unit, suchas the calibration unit 171 in FIG. 7A, according to one embodiment ofthe present disclosure. For ease of illustration, only the relevantcircuit details of the calibration unit 171 are shown in FIG. 10. Asdiscussed in more detail below, the calibration unit 171 may be used torecord the propagation delay of each pixel column. In the embodiment ofFIG. 7A, such recorded values may be provided to the digital block 167to correct the timestamp values captured by the column ADC unit 153 (asrepresented by the Dout line 140). In particular embodiments, thecolumn-specific propagation delays recorded by the calibration unit 171may represent column-specific correction values that may be added orsubtracted, by the digital block 167, to the respective timestamp valuesto obtain corrected timestamp values to be used in constructing the 3Ddepth profile of the object 26 being point-scanned.

As shown in FIG. 10, the calibration unit 171 may include a plurality ofpixels 232-1, 232-2, . . . , 232-N (collectively referred to by thereference numeral “232”) Like the embodiment in FIG. 6, each pixel 232in FIG. 10 is shown to be a 4T PPD pixel. However, in other embodiments,the pixels 232 may be of different types as well—such as 3T pixels, 2Tpixels, and so on, as mentioned before with reference to the discussionof pixels in the embodiment of FIG. 6. Similarly, instead of NMOStransistors, in some embodiments, the pixels 232 may be formed of PMOStransistors or other different types of charge transfer devices. Forease of illustration, biasing and other connection details like thoseshown in FIG. 6 are omitted from the pixel layout shown in FIG. 10.However, using the pixel 232-N as an example, it is noted that, in FIG.10, a Reset (RST) signal may be applied to the gate of each NMOStransistor similar to the transistor 234 and a Transfer (TX) signal maybe applied to the gate of each transistor similar to the transistor 235.The RST and TX signals in the embodiment of FIG. 10 may be similar infunctionality to those signals already discussed before with referenceto FIGS. 6 and 8.

It is noted that, in particular embodiments, the accuracy of timestampcalibration may be increased by making the pixels 232 substantiallyidentical to the pixels in the pixel array 42. This may includefabricating the pixels 232 in the calibration unit 171 and the pixels inthe 2D array 42 using the same semiconductor fabrication process andmaterials so as to minimize structural variances or architecturaldifferences.

In one embodiment, the total number “N” of pixels 232 may equal thenumber of pixels in the “active” portion of a row of pixels in the 2Darray 42. Here, it is assumed that the active portion of each row ofpixels in the 2D array 42 contains the same number of pixels. The“active” portion may refer to those pixels which actually receive lightfrom the object 26 during the laser scan and from which thecorresponding PIXOUT signals are received and processed for 3D depthcalculations. In another embodiment, the value of “N” in FIG. 10 mayequal the total number of columns in the pixel array 42—one calibrationpixel 232 per column. For example, the calibration pixel 232-1 may beused to determine the propagation delay associated the column of pixels102, 105, 108 in FIG. 6, the calibration pixel 232-2 may be used todetermine the propagation delay for the column of pixels 101, 104, and107 in FIG. 6, and so on. As shown in FIG. 10, all of the pixels 232 maybe arranged in a row—similar to a row of pixels in the 2D array 42 inFIG. 6. However, in certain embodiments, such row-based organization maynot be necessary in view of the restricted use of the pixels 232 onlyfor calibration purpose, and not during 3D depth measurements. Thus, therow of pixels 232 may be an extra row of pixels that may not be part ofthe 2D array 42 and that may be used only for generating correctionvalues for timestamp calibration.

In contrast to the embodiments in FIGS. 6 and 8, the entire of row ofpixels 232 may not be selected in the embodiment of FIG. 10 with asingle row selection signal (like the RSEL signal in FIGS. 6 and 8).Instead, each pixel 232 may be individually selected in a controlledmanner using a pixel-specific clocked D-flipflop (DFF). Thus, thecalibration unit 171 is shown to include a plurality of DFFs 237-1,237-2, . . . , 237-N—each DFF providing a respective selection signal(SEL) to its associated pixel. As shown in FIG. 10, there may be “N”individual SEL signals—SEL1 through SELN (which are not individuallyidentified with reference numerals in FIG. 10, but are identified inFIG. 11). As before, using the pixel 232-N as an example, it is notedthat, in FIG. 10, the pixel-specific SEL signal may be applied to thegate of each NMOS transistor similar to the transistor 238. For ease ofillustration, each such transistor is not identified in FIG. 10 using areference numeral. As shown, each DFF may be clocked using a Clock (CLK)input 240. In one embodiment, the CLK input 240 may be the same as theClk input 185 in FIG. 7B. In that case, CLK input 240 may be asystem-wide clock or an image sensor-specific clock generated by the PLLunit 168 (FIG. 7A) or other clock generator (not shown) in the device15. In another embodiment, the CLK signal 240 may be locally-generatedfor the calibration unit 171 and may not be globally available to orshared by other system components. In one embodiment, the frequency ofthe CLK signal 240 may be equal to the frequency of the laser scan—thatis, the frequency with which the laser source 33 (FIG. 2) “interrogates”or point scans the 3D object 26. In another embodiment, the frequency ofthe CLK signal 240 may be different from the laser interrogationfrequency.

In addition to the CLK input 240, the calibration unit 171 may alsoreceive a Set (SET) input 242 to initially set the output of each DFF237 to a logical “high” or “1” value, as discussed below with referenceto FIG. 11. The SET input 242 may be provided by the digital block 167(FIG. 7A) when the timestamp calibration is carried out for thesystem/device 15. In another embodiment, the SET signal 242 may belocally-generated within the calibration unit 171 by appropriate digitallogic (not shown) in the unit 171. As shown, each pixel 232 in thecalibration unit 171 also may receive a VPIX signal 244 for biasing thepixel transistor configuration. Using the pixel 232-N as an example, itis observed that, in FIG. 10, the bias voltage VPIX may be supplied tothe drain terminal of each NMOS transistor similar to the transistor234. In one embodiment, the VPIX signal 244 may be a system-wide supplyvoltage that biases all pixels—the pixels 232 as well as the pixels inthe pixel array 42. In another embodiment, the VPIX voltage 244 may belocally generated—for example, within the calibration unit 171—andapplied to the pixels 232 only.

Upon selection and activation, each pixel 232 may provide apixel-specific output (PIXOUT). In FIG. 10, three such pixel-specificoutputs PIXOUT1, PIXOUT2, and PIXOUTN are identified using referencenumerals 246-248, respectively. These outputs may be processed using thecolumn-specific readout chains 175 to determine the propagation delayfor each column in the pixel array 42. In other words, the pixout input157 to each column-specific readout chain 175 in FIG. 7B may be amultiplexed signal: During routine 2D/3D imaging, the pixout input 157may be a column-specific pixout signal received from a column in thepixel array 42; however, during the timestamp calibration process, thepixout input 157 may be the PIXOUT signal received from one of thepixels 232 associated with the readout chain 175. Thus, the circuithardware of each column-specific readout chain (or ADC unit) 175 may beshared to process a column-specific pixout signal from the columnassociated with the readout chain as well as the PIXOUT signal from oneof the calibration pixels 232 associated with that column. By using thesame readout chains in the column ADC unit 138 (FIG. 6) or 153 (FIG. 7A)as those used to process the pixel outputs from the pixels in the pixelarray 42, the timestamp calibration process may effectively “mimic” or“simulate” each column-specific propagation delay in hardware todetermine the column-specific timestamp correction value.

FIG. 11 is a timing diagram 255 that shows exemplary timing of differentsignals associated with the calibration unit 171 of FIG. 11 to determinecolumn-specific propagation delays according to particular embodimentsof the present disclosure. As discussed below, each pixel-DFF pair inthe calibration unit 171 may be used to process pixel outputs in acontrolled manner to determine calibration pixel-specific—and, hence,the pixel's respective column-specific—propagation delay. Thesecolumn-specific propagation delays may be determined, for example, uponmanufacture of the image sensor unit 24, or at any time when there is achange in the operating temperature or supply voltage beyond apre-defined margin of tolerance. Such temperature or voltage change maybe sensed using appropriate sensor(s) (not shown) that may be includedas part of the system 15. In particular embodiments, the delay values sodetermined may be stored in the image sensor unit 24—for example, in thedigital block 167 (FIG. 7A) of the image sensor unit 24. The storedvalues of column-specific propagation delays may be used later tocorrect/adjust the timestamp values generated for the pixels (in the 2Darray 42) in a column. For example, in the context of FIG. 6, acolumn-specific propagation delay determined for the column of pixels102, 105, 108 may be used to rectify errors in the timestamp valuesgenerated for pixels 102, 105, and 108 during 3D depth measurements. Thesame approach may be used for other columns of pixels.

For ease of discussion, the same reference numerals are used in FIG. 11for the signals, inputs, or outputs that are also shown in FIG. 10. Inparticular embodiments, the propagation delay of each column may berecorded in the following manner:

(1) Initially, the outputs of all DFFs 237 may be set to a logic “high”state (or a binary “1” value) by supplying a high voltage to their Set(“S”) inputs using the SET line 242. The RST and TX signals (not shown)applied to the pixels in the calibration unit 171 may remain highthroughout the calibration process.

(2) After the DFFs 237 are set, the clock signal CLK 240 may be turnedon to clock the output of each pixel 232 for processing through therespective readout chain 175. In one embodiment, the CLK signal 240 maybe an already-running clock and may not need to be turned on, but it maybe applied to the DFFs 237 after they are set. Each calibration pixel232 may be read individually when selected. As a result of the clockingof the DFFs 237, the pixel (or column) select signals—that is, signalsSEL1 through SELN output by the respective DFFs 237—may turn to a logic“low” state (or binary “0” value) one after another. The clockedtransitions of three exemplary SEL signals 258-260 are shown in FIG. 11.In one embodiment, a calibration pixel may be considered “activated”when its respective SEL signal goes low. The timing instant of suchactivation may be referred to as a Start Time (“Ts”). Three such timinginstants Ts1, Ts2, and TsN associated with corresponding SEL signals258-260 are identified in FIG. 11 using reference numerals 262-264,respectively. In one embodiment, each timing instant Ts may be recorded,for example, using the earlier-mentioned global counter approach. Forexample, the high-to-low transition of a pixel-specific SEL signal inFIGS. 10-11 may trigger a memory (not shown) in the calibration unit171, or the line memory 183 in FIG. 7B, or another memory or storageunit (not shown) in the digital block 167 to latch the output of aglobal counter as a value for the corresponding “Ts” parameter. Theclock 185 in FIG. 7B may be used to generate a count value for each Tsinstant—for example, by counting the number of clock pulses occurringbetween the assertion of the SET signal 242 and the high-to-lowtransition of a respective SEL signal. It is observed here that if theCLK signal 240 is a global clock that is also used to generate a countvalue in the global counter and if the frequency of the CLK signal 240equals the laser scan frequency, then there may not be a need to recordthe Ts values because the CLK 240 now mimics the actual laser scanningevents and the laser interrogation frequency already may be known, forexample, by the digital block 167 (FIG. 7A) when the image sensor unit24 is suitably configured by the processor 19 (FIGS. 1-2) to synchronizethe scanning and capturing of light spots.

(3) When a select (SEL) signal output from a DFF 237 goes low (or turns“off”), the corresponding PIXOUT line may go from high to low with aliner droop. As noted before, when a pixel is hit with incident light,its PIXOUT line may start to droop from its high level as indicated bythe downward slopes of three exemplary pixel outputs—the PIXOUT1 signal246, the PIXOUT2 signal 247, and the PIXOUTN signal 248 in FIG. 11. Thehigh level of a pixout signal may represent its reset level in theabsence of detection of light. The PIXOUT1 signal 246 may be receivedfrom the pixel 232-1 and is shown using a dashed line having the pattern“— ⋅⋅ — ⋅⋅ —”. The PIXOUT2 signal 247 may be received from the pixel232-2 and is shown using a straight line. The PIXOUTN signal 248 may bereceived from the pixel 232-N and is shown using a dashed line havingthe pattern “⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅”. The pixel outputs may be generated whenrespective photodiodes detect ambient light from any light source. Thus,during the calibration phase, it may not be necessary to perform thelaser point scan to generate light signals for the photodiodes in thepixels 232. The sequential and clocked generation of PIXOUT signalsthrough controlled generation of SEL signals from respectivepixel-specific DFFs may be considered to “mimic” a laser scan event(where pixel outputs are similarly collected from the pixels in aselected row of pixels in the 2D pixel array 42).

(4) Each PIXOUT signal 246-248 may be compared against a pre-determinedthreshold voltage level in the respective column-specific readout chain175. In the embodiment of FIG. 11, a RAMP signal 266 is shown to providesuch threshold level. The RAMP signal 266 may be the same as the Vrampinput 143 in FIGS. 7B and 8. As mentioned earlier with reference to FIG.8, in particular embodiments, a pre-determined threshold value may beenabled by de-asserting the ramp input (Vramp) 143 to a pre-definedvoltage level. In FIG. 11, the RAMP signal 266 illustrates suchpre-defined voltage level. For each PIXOUT signal 246-248, a comparator(not shown) in the counter unit 181 (FIG. 7B) in the respective readoutchain may compare the PIXOUT signal against the pre-defined threshold ofthe RAMP signal 266 and may detect a time instant—referred here as anEnd Time (“Te”)—when the PIXOUT signal reaches the threshold voltage ofthe RAMP signal 266. In FIG. 11, three such timing instants Te1, Te2,and TeN are identified using reference numerals 268-270, respectively.In particular embodiments, the Te values may be recorded or stored in amemory (not shown) in the calibration unit 171, in the column counter181 or the line memory 183 in FIG. 7B, or another memory or storage unit(not shown) in the digital block 167. It is observed that, in case ofthe timestamp generation approach similar to that shown in FIG. 8, eachtiming instant Te 268-270 may indicate initiation of generation of acorresponding timestamp. On the other hand, in the embodiment where acounting operation is commenced substantially simultaneously withactivation of a calibration pixel or where the counting operation relieson the earlier-mentioned global clock based approach, each timinginstant Te 268-270 may indicate conclusion of generation of thecorresponding timestamp. In the calibration phase, in certainembodiments, any timestamp value(s) generated by a column-specificreadout chain, such as the readout chain 175, may be disregarded.

(5) To obtain the propagation delay of a column, the column-specificvalue of the “Ts” parameter may be subtracted from the column-specificvalue of the “Te” parameter. In other words, in particular embodiments,the column-specific propagation delay for an i^(th) column may be givenby the value of (Tei-Tsi). In particular embodiments, the propagationdelay may be measured as a binary count value or as a number of clockcycles—for example, the clock cycles of a counter clock like the clock185 in FIG. 7B, or a timing value determined with reference to the CLKsignal 240. It is noted here that when a counting operation is commencedsubstantially simultaneously with activation of a calibration pixel 232(for example, using a global counter clock, as noted before) at thetiming instant “Ts”, the counting operation for each calibration pixel232 may be terminated when the corresponding PIXOUT signal reaches theVramp threshold 266—that is, when the respective “Te” instant occurs. Inthat case, the count value generated for each calibration pixel upontermination of the counting operation may provide the column-specificpropagation delay or timestamp correction value for the respectivecolumn of pixels in the 2D pixel array 42. In particular embodiments,the values of column-specific propagation delays—determined as perteachings of the present disclosure—may be stored/recorded in a memorysuch as, for example, a memory or storage unit (not shown) in thedigital block 167 (FIG. 7A). As noted before, these column-specificpropagation delays may be determined, for example, upon manufacture ofthe image sensor unit 24, or at any time when there is a change in theoperating temperature or supply voltage beyond a pre-defined margin oftolerance. In particular embodiments, the above-described approach ofmeasuring column-specific propagation delays may be repeated severaltimes and an average of individual results for a particular pixel columnmay be computed as the value of the column-specific propagation delayfor that pixel column. This average value may be then used forsubsequent timestamp corrections for pixels in that pixel column.

(6) The recorded value of each column-specific propagation delay may beused as a column-specific correction value to calibrate (or correct) thetimestamp of a true laser event during a 3D measurement by a pixel inthe respective column of pixels (in the pixel array 42). As mentionedearlier, in particular embodiments, such timestampcalibration/correction may be performed by the digital logic block 167shown in FIG. 7A. In certain embodiments, the timestamp correction mayinvolve addition (or subtraction) of the recorded value of acolumn-specific propagation delay to a pixel-specific timestamp valuefor a pixel in the respective column (in the pixel array 42). In otherembodiments, the timestamp correction may involve usage of some weightsor scaling factor(s) when applying the recorded value of acolumn-specific propagation delay to a pixel-specific timestamp value.

As mentioned before, in particular embodiments, the frequency of the CLKsignal 240 may be equal to the laser interrogation frequency. In thatcase, the clock signal mimics the actual laser scanning events and,hence, only the value of the “Te” parameter may need to be recorded, forexample, in the column-specific counter 181 (FIG. 7B). On the otherhand, if the frequency of the CLK signal 240 is different from the laserinterrogation frequency, the “Ts” values may be recorded with a globalcounter (not shown) and the “Te” values may be recorded in therespective column counters (like the counter 181 in FIG. 7B). It isobserved that column counters, like the counter 181, may have a limitedbit resolution. Hence, the resulting count value may have a wrappingproblem. In certain embodiments, this problem may be solved by addingone or multiples of the full counts of the column counter. Assuming thecount starting at “0,” the full count of a counter may refer to themaximum count of the counter plus “1”. For example, a 10-bit binarycounter—such as the counter 181 in FIG. 7B—may only count from 0 to 1023in decimal values. Hence, the full count of such a counter is1023+1=1024. The wrapping problem may occur if such a counter withlimited bit resolution is used to count values for the “Ts” and “Te”parameters because, in certain situations, the counter may generate avalue for the “Ts” parameter, reach its full count, and then resetitself before continuing the counting to generate a value for the “Te”parameter. In that case, the counted value of “Ts” may be more than thatof the corresponding “Te” (for example, Ts=1000 and Te=100), therebyproducing a negative result when Ts is subtracted from Te to obtain thepropagation delay. To avoid such a negative result due to the counter'swrapping problem, in one embodiment, the full count of the counter(here, the value of 1024) may be first added to “Te” before the countedvalue of the corresponding “Ts” is subtracted from it. In the examplehere, this addition operation would provide the correct propagationdelay of 1124−1000=124.

FIG. 12 depicts an overall layout of the system 15 in FIGS. 1-2according to one embodiment of the present disclosure. Hence, for easeof reference and discussion, the same reference numerals are used inFIGS. 1-2 and 12 for the common system components/units.

As discussed earlier, the imaging module 17 may include the hardwareshown in the exemplary embodiments of FIGS. 2, 6, 7A-7B, and 10 toaccomplish 2D imaging, 3D depth measurements, and timestamp calibrationas per the inventive aspects of the present disclosure. The processor 19may be configured to interface with a number of external devices. In oneembodiment, the imaging module 17 may function as an input device thatprovides data inputs—in the form of pixel event data such as, forexample, the processed data output 170 in FIG. 7A—to the processor 19for further processing. The processor 19 may also receive inputs fromother input devices (not shown) that may be part of the system 15. Someexamples of such input devices include a computer keyboard, a touchpad,a touch-screen, a joystick, a physical or virtual “clickable button,”and/or a computer mouse/pointing device. In FIG. 12, the processor 19 isshown coupled to the system memory 20, a peripheral storage unit 274,one or more output devices 276, and a network interface unit 278. InFIG. 12, a display unit is shown as an output device 276. In someembodiments, the system 15 may include more than one instance of thedevices shown. Some examples of the system 15 include a computer system(desktop or laptop), a tablet computer, a mobile device, a cellularphone, a video gaming unit or console, a machine-to-machine (M2M)communication unit, a robot, an automobile, a virtual reality equipment,a stateless “thin” client system, a car's dash-cam or rearview camerasystem, or any other type of computing or data processing device. Invarious embodiments, all of the components shown in FIG. 12 may behoused within a single housing. Thus, the system 15 may be configured asa standalone system or in any other suitable form factor. In someembodiments, the system 15 may be configured as a client system ratherthan a server system.

In particular embodiments, the system 15 may include more than oneprocessor (e.g., in a distributed processing configuration). When thesystem 15 is a multiprocessor system, there may be more than oneinstance of the processor 19 or there may be multiple processors coupledto the processor 19 via their respective interfaces (not shown). Theprocessor 19 may be a System on Chip (SoC) and/or may include more thanone Central Processing Units (CPUs).

As mentioned earlier, the system memory 20 may be anysemiconductor-based storage system such as, for example, DRAM, SRAM,PRAM, RRAM, CBRAM, MRAM, STT-MRAM, and the like. In some embodiments,the memory unit 20 may include at least one 3DS memory module inconjunction with one or more non-3DS memory modules. The non-3DS memorymay include Double Data Rate or Double Data Rate 2, 3, or 4 SynchronousDynamic Random Access Memory (DDR/DDR2/DDR3/DDR4 SDRAM), or Rambus®DRAM, flash memory, various types of Read Only Memory (ROM), etc. Also,in some embodiments, the system memory 20 may include multiple differenttypes of semiconductor memories, as opposed to a single type of memory.In other embodiments, the system memory 20 may be a non-transitory datastorage medium.

The peripheral storage unit 274, in various embodiments, may includesupport for magnetic, optical, magneto-optical, or solid-state storagemedia such as hard drives, optical disks (such as Compact Disks (CDs) orDigital Versatile Disks (DVDs)), non-volatile Random Access Memory (RAM)devices, and the like. In some embodiments, the peripheral storage unit274 may include more complex storage devices/systems such as disk arrays(which may be in a suitable RAID (Redundant Array of Independent Disks)configuration) or Storage Area Networks (SANs), and the peripheralstorage unit 274 may be coupled to the processor 19 via a standardperipheral interface such as a Small Computer System Interface (SCSI)interface, a Fibre Channel interface, a Firewire® (IEEE 1394) interface,a Peripheral Component Interface Express (PCI Express™) standard basedinterface, a Universal Serial Bus (USB) protocol based interface, oranother suitable interface. Various such storage devices may benon-transitory data storage media.

The display unit 276 may be an example of an output device. Otherexamples of an output device include a graphics/display device, acomputer screen, an alarm system, a CAD/CAM (Computer AidedDesign/Computer Aided Machining) system, a video game station, asmartphone display screen, or any other type of data output device. Insome embodiments, the input device(s), such as the imaging module 17,and the output device(s), such as the display unit 276, may be coupledto the processor 19 via an I/O or peripheral interface(s).

In one embodiment, the network interface 278 may communicate with theprocessor 19 to enable the system 15 to couple to a network (not shown).In another embodiment, the network interface 278 may be absentaltogether. The network interface 278 may include any suitable devices,media and/or protocol content for connecting the system 15 to anetwork—whether wired or wireless. In various embodiments, the networkmay include Local Area Networks (LANs), Wide Area Networks (WANs), wiredor wireless Ethernet, telecommunication networks, or other suitabletypes of networks.

The system 15 may include an on-board power supply unit 280 to provideelectrical power to various system components illustrated in FIG. 12.The power supply unit 280 may receive batteries or may be connectable toan AC electrical power outlet. In one embodiment, the power supply unit280 may convert solar energy or other renewable energy into electricalpower.

In one embodiment, the imaging module 17 may be integrated with ahigh-speed interface such as, for example, a Universal Serial Bus 2.0 or3.0 (USB 2.0 or 3.0) interface or above, that plugs into any PersonalComputer (PC) or laptop. A non-transitory, computer-readable datastorage medium, such as, for example, the system memory 20 or aperipheral data storage unit such as a CD/DVD may store program code orsoftware. The processor 19 and/or the digital processing block 167 (FIG.7A) in the imaging module 17 may be configured to execute the programcode, whereby the device 15 may be operative to perform the 2D imagingand 3D depth measurements (and related timestamp calibration) asdiscussed hereinbefore—such as, for example, the operations discussedearlier with reference to FIGS. 1-11. For example, in certainembodiments, upon execution of the program code, the processor 19 and/orthe digital block 167 may suitably configure (or activate) relevantcircuit components—such as the calibration unit 171 and the readoutchain 175—to appropriately carry out the timestamp calibration as perteachings of the present disclosure with the help of those components.The program code or software may be proprietary software or open sourcesoftware which, upon execution by the appropriate processing entity—suchas the processor 19 and/or the digital block 167—may enable theprocessing entity to perform timestamp calibration, capture pixel eventsusing their precise timing, process them, render them in a variety offormats, and display them in the 2D and/or 3D formats. As noted earlier,in certain embodiments, the digital processing block 167 in the imagingmodule 17 may perform some of the processing of pixel event signalsbefore the pixel output data are sent to the processor 19 for furtherprocessing and display. In other embodiments, the processor 19 may alsoperform the functionality of the digital block 167, in which case, thedigital block 167 may not be a part of the imaging module 17.

In the preceding description, for purposes of explanation and notlimitation, specific details are set forth (such as particulararchitectures, waveforms, interfaces, techniques, etc.) in order toprovide a thorough understanding of the disclosed technology. However,it will be apparent to those skilled in the art that the disclosedtechnology may be practiced in other embodiments that depart from thesespecific details. That is, those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the disclosed technology. In someinstances, detailed descriptions of well-known devices, circuits, andmethods are omitted so as not to obscure the description of thedisclosed technology with unnecessary detail. All statements hereinreciting principles, aspects, and embodiments of the disclosedtechnology, as well as specific examples thereof, are intended toencompass both structural and functional equivalents thereof.Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, such as, for example, any elements developed that perform thesame function, regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat block diagrams herein (e.g., in FIGS. 1-2) can represent conceptualviews of illustrative circuitry or other functional units embodying theprinciples of the technology. Similarly, it will be appreciated that theflowcharts in FIGS. 3 and 9 represent various processes which may besubstantially performed by a processor (e.g., the processor 19 in FIG.12 and/or the digital block 167 in FIG. 7A). Such processor may include,by way of example, a general purpose processor, a special purposeprocessor, a conventional processor, a digital signal processor (DSP), aplurality of microprocessors, one or more microprocessors in associationwith a DSP core, a controller, a microcontroller, Application SpecificIntegrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs)circuits, any other type of integrated circuit (IC), and/or a statemachine. Some or all of the functionalities described above in thecontext of FIGS. 1-11 also may be provided by such processor, in thehardware and/or software.

When certain inventive aspects require software-based processing, suchsoftware or program code may reside in a computer-readable data storagemedium. As noted earlier, such data storage medium may be part of theperipheral storage 274, or may be part of the system memory 20 or anyinternal memory (not shown) of the image sensor unit 24, or theprocessor's 19 internal memory (not shown). In one embodiment, theprocessor 19 or the digital block 167 may execute instructions stored onsuch a medium to carry out the software-based processing. Thecomputer-readable data storage medium may be a non-transitory datastorage medium containing a computer program, software, firmware, ormicrocode for execution by a general purpose computer or a processormentioned above. Examples of computer-readable storage media include aROM, a RAM, a digital register, a cache memory, semiconductor memorydevices, magnetic media such as internal hard disks, magnetic tapes andremovable disks, magneto-optical media, and optical media such as CD-ROMdisks and DVDs.

Alternative embodiments of the imaging module 17 or the system 15comprising such an imaging module according to inventive aspects of thepresent disclosure may include additional components responsible forproviding additional functionality, including any of the functionalityidentified above and/or any functionality necessary to support thesolution as per the teachings of the present disclosure. Althoughfeatures and elements are described above in particular combinations,each feature or element can be used alone without the other features andelements or in various combinations with or without other features. Asmentioned before, various 2D and 3D imaging functions discussed hereinmay be provided through the use of hardware (such as circuit hardware)and/or hardware capable of executing software/firmware in the form ofcoded instructions or microcode stored on a computer-readable datastorage medium (mentioned above). Thus, such functions and illustratedfunctional blocks are to be understood as being eitherhardware-implemented and/or computer-implemented, and thusmachine-implemented.

The foregoing describes a system and method in which the same imagesensor—that is, all of the pixels in the image sensor—may be used tocapture both a 2D image of a 3D object and 3D depth measurements for theobject. The image sensor may be part of a camera in a mobile device suchas, for example, a smartphone. A laser light laser source may be used topoint scan the surface of the object with light spots, which may be thendetected by a pixel array in the image sensor to generate the 3D depthprofile of the object using triangulation. In the 3D mode, the laser mayproject a sequence of light spots on the surface of the object along ascan line. The illuminated light spots may be detected using a row ofpixels in the pixel array such that the row forms an epipolar line ofthe scan line. The detected light spots may be timestamped to remove anyambiguity in triangulation and, hence, to reduce the amount of depthcomputation and system power. A timestamp may also provide acorrespondence between the pixel location of a captured laser spot andthe respective scan angle of the laser light source to determine depthusing triangulation. An ADC unit in the image sensor may operate as aTime-to-Digital (TDC) converter to generate timestamps. To improve theperformance of the 3D camera system, a timestamp calibration circuit maybe provided on-board to record the propagation delay of each column ofpixels in the pixel array and to provide necessary corrections to thetimestamp values generated during 3D depth measurements.

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a wide range of applications. Accordingly, the scope of patentedsubject matter should not be limited to any of the specific exemplaryteachings discussed above, but is instead defined by the followingclaims.

What is claimed is:
 1. An imaging unit, comprising: a light source thatpoint scans an object along a scanning line, the point scan comprising asequence of light spots projected toward a surface of the object by thelight source; and an image sensor unit comprising: a pixel array formingan image plane, the pixel array including at least one row of pixelsthat forms at least a portion of an epipolar line of the scanning line,each pixel in the at least one row of pixels being associated with acorresponding column in the pixel array, and each pixel in the at leastone row of pixels generating a pixel-specific output in response todetecting a light spot in the sequence of light spots that has beenreflected from the surface of the object; and a timestamp generatorassociated with a corresponding pixel in the at least one row of pixels,the timestamp generator generating a pixel-specific timestamp value forthe pixel-specific output of the corresponding pixel.
 2. The imagingunit of claim 1, further comprising a processing unit coupled to atleast one timestamp generator, the processing unit generating acorrected timestamp value for the pixel-specific timestamp value for thepixel-specific output of the corresponding pixel based on acolumn-specific correction value representing a column-specificpropagation delay between the pixel-specific output and when thepixel-specific output reaches a predetermined threshold.
 3. The imagingunit of claim 2, wherein the processing unit further determines adistance to the detected light spot on the surface of the object basedat least on the corrected timestamp value and on a scan angle used bythe light source to project the detected light spot.
 4. The imaging unitof claim 2, further comprising a calibration unit coupled to the atleast one timestamp generator, the calibration unit comprising acalibration pixel corresponding to a column of the at least one row ofpixels and generating a calibration output for the corresponding column.5. The imaging unit of claim 4, wherein each calibration unit records afirst timing instant when the corresponding calibration pixel isactivated to generate the calibration output and records a second timinginstant when the calibration output reaches the predetermined threshold;and wherein the calibration unit is further coupled to the processingunit and the processing unit subtracts a first timing instant from acorresponding second timing instant to generate the column-specificcorrection value for the corresponding column.
 6. The imaging unit ofclaim 5, wherein the calibration unit clocks the calibration output ofeach calibration pixel using a clock frequency that is equal to a scanfrequency used by the light source to project lights spots of the pointscan.
 7. The imaging unit of claim 5, wherein the calibration unitclocks the calibration output of each the calibration pixel using aclock frequency that is different from a scan frequency used by thelight source to project lights spots of the point scan.
 8. The imagingunit of claim 5, wherein at least one timestamp generator is operativebegins a counting operation substantially simultaneously with anactivation of the corresponding calibration pixel and terminates thecounting operation when the output of the calibration pixel reaches thepredetermined threshold; and wherein the calibration unit is furthercoupled to the processing unit and the processing unit selects a countvalue generated upon termination of the counting operation as thecolumn-specific correction value corresponding to the column.
 9. Asystem, comprising: a light source that point scans an object along ascanning line, the point scan comprising a sequence of light spotsprojected toward a surface of the object by the light source; a pixelarray forming an image plane, the pixel array including at least one rowof pixels that forms at least a portion of an epipolar line of thescanning line, each pixel in the at least one row of pixels beingassociated with a corresponding column in the pixel array, and eachpixel in the at least one row of pixels generating a pixel-specificoutput in response to detecting a light spot in the sequence of lightspots that has been reflected from the surface of the object; atimestamp generator that is associated with a corresponding pixel in theat least one row of pixels, the timestamp generator generating apixel-specific timestamp value for the pixel-specific output of thecorresponding pixel; and a processor coupled to at least one timestampgenerator, the processor generating a corrected timestamp value for thepixel-specific timestamp value for the pixel-specific output of thecorresponding pixel based on a column-specific correction valuerepresenting a column-specific propagation delay between thepixel-specific output and when the pixel-specific output reaches apredetermined threshold.
 10. The system of claim 9, wherein theprocessor determines a distance to the detected light spot on thesurface of the object based at least on the corrected timestamp valueand on a scan angle used by the light source to project the detectedlight spot.
 11. The system of claim 10, further comprising a calibrationunit coupled to at least one timestamp generator, the calibration unitcomprising a plurality of calibration pixels, each calibration pixelcorresponding to a column of the at least one row of pixels andgenerating a calibration output for the corresponding column.
 12. Thesystem of claim 11, wherein the calibration unit is further coupled tothe processor, wherein the processor further configures the calibrationunit to activate at least one of the calibration pixels to output acalibration output, wherein at least one timestamp generatorcorresponding to the at least one calibration pixel records a firsttiming instant when the at least one calibration pixel is activated andrecords a second timing instant when the calibration output of the atleast one calibration pixel reaches the predetermined threshold, whereinthe processor subtracts the first timing instant from the second timinginstant to obtain the column-specific correction value for the columncorresponding to the at least one calibration pixel, and wherein atleast one timestamp generator begins generation of a correspondingtimestamp upon occurrence of the first timing instant and stopsgeneration of the corresponding timestamp upon occurrence of the secondtiming instant.
 13. The system of claim 12, wherein the calibration unitclocks the calibration output of each calibration pixel using a clockfrequency that is sustainably equal to a scan frequency used by thelight source to project lights spots of the point scan.
 14. The systemof claim 12, wherein the calibration unit clocks the calibration outputof each the calibration pixel using a clock frequency that is differentfrom a scan frequency used by the light source to project lights spotsof the point scan.
 15. An image sensor unit, comprising: a pixel arrayforming an image plane, the pixel array including at least one row ofpixels that forms at least a portion of an epipolar line of a scanningline, the scanning comprising a sequence of light spots projected towarda surface of an object by a light source, each pixel in the at least onerow of pixels being associated with a corresponding column in the pixelarray, and each pixel in the at least one row of pixels generating apixel-specific output in response to detecting a light spot in thesequence of light spots that has been reflected from the surface of theobject; a plurality of analog-to-digital Converter (ADC) units, each ADCunit being associated with a corresponding pixel in the at least one rowof pixels and generating a pixel-specific timestamp value for thepixel-specific output of the corresponding pixel; and a calibration unitcoupled to the plurality of ADC units, the calibration unit comprising aplurality of calibration pixels, each calibration pixel corresponding toa column of the at least one row of pixels and generating a calibrationoutput for the corresponding column.
 16. The image sensor unit of claim15, wherein each corresponding ADC unit records a first timing instantwhen the corresponding calibration pixel is activated to generate thecalibration output and records a second timing instant when thecalibration output reaches a predetermined threshold; and wherein thecalibration unit is further coupled to a processing unit and theprocessing unit subtracts each first timing instant from the secondtiming instant to generate a column-specific correction value for thecorresponding column.
 17. The image sensor unit of claim 16, wherein thecalibration unit clocks the calibration output of each calibration pixelusing a clock frequency that is equal to a scan frequency used by thelight source to project lights spots of the point scan.
 18. The imagesensor unit of claim 16, wherein the calibration unit clocks thecalibration output of each the calibration pixel using a clock frequencythat is different from a scan frequency used by the light source toproject lights spots of the point scan.
 19. The image sensor unit ofclaim 16, wherein at least one of the plurality of ADC units isoperative begins a counting operation substantially simultaneously withan activation of the corresponding calibration pixel and terminates thecounting operation when the output of the calibration pixel reaches thepredetermined threshold; and wherein the calibration unit is furthercoupled to the processing unit and the processing unit selects a countvalue generated upon termination of the counting operation as thecolumn-specific correction value corresponding to the column.
 20. Theimage sensor unit of claim 15, further comprising a processing unitcoupled to the plurality of ADC units, the processing unit generating acorrected timestamp value for the pixel-specific timestamp value for thepixel-specific output of the corresponding pixel based on acolumn-specific correction value representing a column-specificpropagation delay between the pixel-specific output and when thepixel-specific output reaches a predetermined threshold, wherein theprocessing unit further determines a distance to the detected light spoton the surface of the object based at least on the corrected timestampvalue and on a scan angle used by the light source to project thedetected light spot.